Hi aisv,
You can try the following solutions:
1/ Maybe this override from the old version of T3 framework, you can try to rename this folder:
JROOT/plugins/system/t4/base-bs3/html/com_content/form/
and see how it goes as the new version doesn't override this form anymore.
2/ You can access this 'edit' folder:
JROOT/plugins/system/t3/base-bs3/html/layouts/joomla/edit/
rename all files except for this one: frontediting_modules.php
The issue here is that in the new version, some override files (like above) are not required anymore but they will not to be removed manually. If you renamed them already, everything will work fine.
Hope this helps!