The file ft.forms_linked.php looks like it may be from a custom addon, most probably https://addons.reinos.nl/email-form-linked
The solution is to update the addon so it’s EE5 compatible, but I noted that it’s for EE2 only, maybe contact the developer to see if it’s going to be updated.
Another solution is to delete the offending channel field and replace with a native field (backup your DB first!)
If you were using the fieldtype in any of your channels it may still be listed in the database as a fieldtype, when you create/edit entries EE may be looking for it but can’t find it - hence the error.
If you used that fields tag in a template the error should only display on public pages/templates.
Yes, I find that ft.forms_linked.php was part of an addon called ‘forms_linked’. The problem occurs only with channels and field groups when I try to open it for editing, so there is no problem with templates. I uploaded the forms_linked to the server in /users/addons/ and now I can open and create channels but with warning errors. How do I get ridd off of that ft.forms_linked in the database? Thanks