giving /var/www/site1 to www-data:www-data crashes all WP sites on server - linux

Question

-- See Tl;dr below for a short version --
On my ubuntu-16.04 droplet apache2 and php7 both use the user www-data. At some point all three installations of wordpress-4.7.2 seemingly without reason started asking for FTP credentials, indicating they don't have sufficient rights. Each wp instance has it's own mysql database.
I tried to solve this for /site1 with
sudo chown www-data:www-data /var/www/site1/* -R
which shouldn't change anything as this is how the permissions were set already. For whatever reason this caused the browser to return a
HTTP ERROR 500
for the sites in /var/www/site1, /var/www/site2 and /var/www/site3 – nothing works anymore.
The only way I've found to get out of this is to restore the droplet. But each time I try to get the permissions right, I end up with all sites down again.
These are the current permission settings:
drwxr-xr-x 14 root root 4096 Feb 3 XX:14 /var/
drwxrwxr-x 8 www-data www-data 4096 Mar 5 XX:27 /var/www/
drwxr-sr-x 3 www-data www-data 4096 Mar 5 XX:13 /var/www/site1/
drwxrwxr-x 3 www-data www-data 4096 Feb 25 XX:51 /var/www/site2/
drwxrwxr-x 3 www-data www-data 4096 Feb 28 XX:06 /var/www/site3/
The sudo user is member of www-data:
user1#droplet:~$ members www-data
www-data user2 user1
A freshly installed Theme on /site1 caused dozens of PHP Fatal errors like this one:
[Sun Mar 05 19:24:04.003189 2017] [:error] [pid 5632] [client 31.10.138.238:50870]
PHP Fatal error: Uncaught Error: Call to undefined function mysql_escape_string()
in /var/www/site1/html/wp-content/themes/gloria/functions.php:60\nStack trace:\n#0
/var/www/site1/html/wp-settings.php(425): include()\n#1 /var/www/bw/html/wp-config.php(89):
require_once('/var/www/site1/htm...')\n#2 /var/www/bw/html/wp-load.php(37):
require_once('/var/www/site1/htm...')\n#3 /var/www/bw/html/wp-admin/admin.php(31):
require_once('/var/www/site1/htm...')\n#4 /var/www/bw/html/wp-admin/themes.php(10):
require_once('/var/www/site1/htm...')\n#5 {main}\n thrown in
/var/www/site1/html/wp-content/themes/gloria/functions.php on line 60
After deleting the theme, /site1 went back online. I have no idea why. The first two times I ended up restoring the droplet because of
HTTP ERROR 500
this theme wasn't involved. Even though /site1 is back up, /site2 and /site3 remain stuck with
HTTP ERROR 500
Since the removal of the theme in /site1 the Apache Error Log doesn't have any suspicious entries:
[Sun Mar 05 19:56:35.456584 2017] [mpm_prefork:notice] [pid 1671] AH00171: Graceful restart requested, doing restart
[Sun Mar 05 19:56:35.662742 2017] [mpm_prefork:notice] [pid 1671] AH00163: Apache/2.4.18 (Ubuntu) configured -- resuming normal operations
[Sun Mar 05 19:56:35.662765 2017] [core:notice] [pid 1671] AH00094: Command line: '/usr/sbin/apache2'
Any ideas?
Edit: An instance of ActiveCollab in /var/www/site2/activecollab/ never stopped working during the whole issue. All connection and permission checks done by ActiveCollab for it's own files, folders and database are positive.
Tl;dr: WP1 asks for FTP, I say
sudo chown www-data:www-data /var/www/site1/* -R
WP1, WP2 and WP3 tell the browser to
HTTP ERROR 500
while ActiveCollab in a subfolder of WP2 doesn't give a sh*t and keeps running.

Answer 1

Try these
sudo chmod -R 774 /var/www/yourwordpressfolder
And then
sudo chown -R www-data:www-data /var/www/yourwordpress
Finally
sudo chmod -R 777 /var/www/yourwordpressfolder