Can't import emails

I am using Version 7.12.5 Sugar Version 6.5.25 (Build 344). It is a dockerized bitnami distribution, which other than this issue seems to be working fine.
When I try to import an email I just get a message as shown in the screen capture:

I get no error messages in suitecrm.log if I set log level to Error in the settings.
Any suggestion on what could be going on?