After upgrading from Moodle 3.4 to 3.4+, all steps succeeded but many functions of the site administration started to send an exception (500 internal error).
After investigating with my hosting provider, the exception was due to a permission mismatch. Apparently, the files in the admin folders have become writable (664) for group and should only be set to 644.
This was fixed with changing permissions as follow :
find . -type f ! -perm 0664 | xargs chmod 644
thanks. happy new year to everyone