Odoo 11 throws error on customer/partner listing after upgrade from odoo 10 - python-3.x

Question

I have recently upgraded an odoo 10 instance to odoo 11 using an open upgrade. After deleting projects module (apparently it cannot be properly migrated) and fixing some custom module I finally do have a running odoo 11 instance. However, whenever I try to click on customers/vendors odoo throws this error:
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/odoo/fields.py", line 948, in __get__
value = record.env.cache.get(record, self)
File "/usr/lib/python3/dist-packages/odoo/api.py", line 977, in get
value = self._data[key][field][record._ids[0]]
KeyError: 421
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/odoo/fields.py", line 948, in __get__
value = record.env.cache.get(record, self)
File "/usr/lib/python3/dist-packages/odoo/api.py", line 977, in get
value = self._data[key][field][record._ids[0]]
KeyError: 421
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/odoo/http.py", line 653, in _handle_exception
return super(JsonRequest, self)._handle_exception(exception)
File "/usr/lib/python3/dist-packages/odoo/http.py", line 312, in _handle_exception
raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
File "/usr/lib/python3/dist-packages/odoo/tools/pycompat.py", line 87, in reraise
raise value
File "/usr/lib/python3/dist-packages/odoo/http.py", line 695, in dispatch
result = self._call_function(**self.params)
File "/usr/lib/python3/dist-packages/odoo/http.py", line 344, in _call_function
return checked_call(self.db, *args, **kwargs)
File "/usr/lib/python3/dist-packages/odoo/service/model.py", line 97, in wrapper
return f(dbname, *args, **kwargs)
File "/usr/lib/python3/dist-packages/odoo/http.py", line 337, in checked_call
result = self.endpoint(*a, **kw)
File "/usr/lib/python3/dist-packages/odoo/http.py", line 939, in __call__
return self.method(*args, **kw)
File "/usr/lib/python3/dist-packages/odoo/http.py", line 517, in response_wrap
response = f(*args, **kw)
File "/usr/lib/python3/dist-packages/odoo/addons/web/controllers/main.py", line 934, in call_kw
return self._call_kw(model, method, args, kwargs)
File "/usr/lib/python3/dist-packages/odoo/addons/web/controllers/main.py", line 926, in _call_kw
return call_kw(request.env[model], method, args, kwargs)
File "/usr/lib/python3/dist-packages/odoo/api.py", line 697, in call_kw
return call_kw_model(method, model, args, kwargs)
File "/usr/lib/python3/dist-packages/odoo/api.py", line 682, in call_kw_model
result = method(recs, *args, **kwargs)
File "/usr/lib/python3/dist-packages/odoo/models.py", line 1296, in load_views
for [v_id, v_type] in views
File "/usr/lib/python3/dist-packages/odoo/models.py", line 1296, in <dictcomp>
for [v_id, v_type] in views
File "/usr/lib/python3/dist-packages/odoo/addons/mail/models/mail_thread.py", line 374, in fields_view_get
res = super(MailThread, self).fields_view_get(view_id=view_id, view_type=view_type, toolbar=toolbar, submenu=submenu)
File "/usr/lib/python3/dist-packages/odoo/models.py", line 1375, in fields_view_get
result = self._fields_view_get(view_id=view_id, view_type=view_type, toolbar=toolbar, submenu=submenu)
File "/usr/lib/python3/dist-packages/odoo/addons/base/res/res_partner.py", line 318, in _fields_view_get
res = super(Partner, self)._fields_view_get(view_id=view_id, view_type=view_type, toolbar=toolbar, submenu=submenu)
File "/usr/lib/python3/dist-packages/odoo/models.py", line 1338, in _fields_view_get
root_view = View.browse(view_id).read_combined(['id', 'name', 'field_parent', 'type', 'model', 'arch'])
File "/usr/lib/python3/dist-packages/odoo/addons/base/ir/ir_ui_view.py", line 730, in read_combined
arch = self.apply_view_inheritance(arch_tree, root.id, self.model)
File "/usr/lib/python3/dist-packages/odoo/addons/base/ir/ir_ui_view.py", line 674, in apply_view_inheritance
sql_inherit = self.get_inheriting_views_arch(source_id, model)
File "/usr/lib/python3/dist-packages/odoo/addons/base/ir/ir_ui_view.py", line 498, in get_inheriting_views_arch
for view in views.sudo()
File "/usr/lib/python3/dist-packages/odoo/addons/base/ir/ir_ui_view.py", line 499, in <listcomp>
if not view.groups_id or (view.groups_id & user_groups)]
File "/usr/lib/python3/dist-packages/odoo/fields.py", line 952, in __get__
self.determine_value(record)
File "/usr/lib/python3/dist-packages/odoo/fields.py", line 1065, in determine_value
self.compute_value(recs)
File "/usr/lib/python3/dist-packages/odoo/fields.py", line 1019, in compute_value
self._compute_value(records)
File "/usr/lib/python3/dist-packages/odoo/fields.py", line 1010, in _compute_value
getattr(records, self.compute)()
File "/usr/lib/python3/dist-packages/odoo/addons/base/ir/ir_ui_view.py", line 260, in _compute_arch
view.arch = pycompat.to_text(arch_fs or view.arch_db)
File "/usr/lib/python3/dist-packages/odoo/fields.py", line 952, in __get__
self.determine_value(record)
File "/usr/lib/python3/dist-packages/odoo/fields.py", line 1055, in determine_value
record._prefetch_field(self)
File "/usr/lib/python3/dist-packages/odoo/models.py", line 2663, in _prefetch_field
result = records.read([f.name for f in fs], load='_classic_write')
File "/usr/lib/python3/dist-packages/odoo/models.py", line 2601, in read
self._read_from_database(stored, inherited)
File "/usr/lib/python3/dist-packages/odoo/models.py", line 2744, in _read_from_database
values[index] = translate(ids[index], values[index])
File "/usr/lib/python3/dist-packages/odoo/fields.py", line 1344, in translate
return self.translate(src_trans.get, value)
File "/usr/lib/python3/dist-packages/odoo/tools/translate.py", line 306, in xml_translate
try:
File "/usr/lib/python3/dist-packages/odoo/tools/translate.py", line 284, in parse_xml
return etree.fromstring(bytes(text, encoding='utf-8'))
File "src/lxml/lxml.etree.pyx", line 3213, in lxml.etree.fromstring (src/lxml/lxml.etree.c:79003)
File "src/lxml/parser.pxi", line 1843, in lxml.etree._parseMemoryDocument (src/lxml/lxml.etree.c:118275)
ValueError: Unicode strings with encoding declaration are not supported. Please use bytes input or XML fragments without declaration.
this seems pretty odd since whatever I read suggests to really change the odoo code itself, which I feel I should definitely not do. However, does anyone have a direction on where to look for this issue? I see that in translate.py there is an xml_parse without any encoding, but if this would be the root cause odoo itself shouldn't be working at all, but it works on a fresh instance, so the root cause must be something else then.
Thanks for any hint.
UPDATE: some further details
what i was trying to achieve:
I run an instance of odoo 10 and wante to upgrade it to odoo 11 using https://github.com/OCA/OpenUpgrade. This worked in the sense that all data was migrated and I had a working odoo 11 instance running. Before upgrading i stripped the odoo 10 instance to the bare minimum of modules to not infer any further conflicts/issues while upgrading. Si after upgrading i could log in and all looked good, except when i click on any view that is supposed to display customer data. There i get the error message as describe above. It seems there is some problem during upgrading that causes this issue. But i have no idea what this could be. I though i could export & delete all customers in the old instance, upgrade and import those, but this is of course not possible since there are documents/invoices attached to the customers. So this is not an option...
any ideas?