Everything was working fine in version 8 but have upgraded to 11. Now can't fetch emails from my server.

This is the error in my log file:

Traceback (most recent call last):

  File "/odoo/odoo-server/addons/fetchmail/models/fetchmail.py", line 207, in fetch_mail

    message = '\n'.join(messages)

TypeError: sequence item 0: expected str instance, bytes found

Any help please?