It's not the /tmp folder in mailwizz app, there's no such thing. It's about the tmp folder of your PHP instance.
Go to Backend -> Misc -> PHP Info and search in page after upload_tmp_dir. If it has no value, then it means it will fallback on /tmp folder.
This just crossed my mind, you said you fixed your DB problem by connecting directly to an ip address because the sock file was not created. This indicates a serious file system issue on that server. Now PHP complaints about not being able to write in temporary folder, which is again a filesystem problem.
Doesn't this raise any questions about your hosting?