Seting up a "language for programms not supporting unicode" only partialy solved MS Access' "problem communicating with OLE server or ActiveX control"

Question

While trying to migrate an Access 2003 mdb database to Access 2021, I got an error that there was a "problem communicating with OLE server or ActiveX control"

After a web search I changed the "Language for programms not supporting unicode" in the settings from English (US) to Greek.

That only partially solved the problem. Some actions (like clicking buttons to open forms) started working again but I noticed that:

1. Some of the labels on the forms show greek leters properly, while others don't (they display the 1252 code page equivalent characters.
2. Some fields diplay #name instead of the field value as if the data source field has not been conected to the form field properly.
3. In the forms above, the user gets the "problem communicating with OLE server or ActiveX control" when trying to open a new form based on the information of the original form

The database was on Windows 7 and the new PC is running Windows 11.

It bufles me that the language setting worked for some objects of the database and not for others. Is there something I can do to make the database run correctly in the new environment?