Troubleshoot gcc installation - linux

Question

I have the following setup:
I don't have root priviliges, so I usually build my stuff from source to $HOME/.local
System is Debian 3.16.84-1
I wanted to update my gcc installation.
there's version 4.9.2 installed systemwide, and I installed version 8.4.0 some years ago for local user.
Now I try to install the current version 11.2.0
My first try was to do it with version 8.4.0
the following error occurs when running make:
if [ x"-fpic" != x ]; then \
gcc -c -DHAVE_CONFIG_H -g -I. -I../.././libiberty/../include -W -Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -pedantic -D_GNU_SOURCE -fpic ../.././libiberty/floatformat.c -o
pic/floatformat.o; \
else true; fi
*** Error in `/home/user/.local/libexec/gcc/x86_64-pc-linux-gnu/8.4.0/cc1': free(): invalid next size (fast): 0x0000000002a19c70 ***
../.././libiberty/floatformat.c: In function ‘floatformat_to_double’:
../.././libiberty/floatformat.c:547:2: internal compiler error: Aborted
dto = ldexp (1.0, exponent);
^~~
using 8.9.2 I get a similar error:
if [ x"-fpic" != x ]; then \
gcc -c -DHAVE_CONFIG_H -g -I. -I../.././libiberty/../include -W -Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -pedantic -D_GNU_SOURCE -fpic ../.././libiberty/floatformat.c -o pic/floatformat.o; \
else true; fi
*** Error in `/usr/lib/gcc/x86_64-linux-gnu/4.9/cc1': free(): invalid next size (fast): 0x0000000003626810 ***
*** Error in `/usr/lib/gcc/x86_64-linux-gnu/4.9/cc1': malloc(): memory corruption: 0x0000000003626850 ***
gcc: internal compiler error: Aborted (program cc1)
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-4.9/README.Bugs> for instructions.
Makefile:804: recipe for target 'floatformat.o' failed
I have the feeling there's a more general underlying problem somewhere but I have no idea how to troubleshoot.
I can post full logs if needed.