There may be differences in settings between the new and the old server.
I’ve found the most reliable way to change servers, do a clean Grav install on the new server first.
- Back up the old ‘user’ folder
- extract clean Grav to the appropriate folder.
- Use (temporary login and run the setup
- rename new user folder e.g to user.old
- restore your site user folder (original login will work)
Hope this helps