Glad to hear that your issue is resolved - sort of.

I also have issues with TB and POP3. Phenomenon is that initially after boot and / or DSL connect of the router a few POP exchanges work OK but somewhere later on I get an error pop up window saying that GXM replied “” (nothing, empty string).

I didn’t isolate the issue yet but I believe that it has to do with combination of Thunderbird and Avast Email scanner because other systems in the same network which do not have Avast installed do not exhibit this behavior.

Settings:
TB: POP3, port 110 without security
AVast Mail scanner: pop.gmx.net set to SSL/TLS (port 995)