This means you have something messing with your network connection. Turn off all VPNs, ad blockers, privacy blockers, blockada, PiHole, etc.
From localhost (127.0.0.1) to localhost? What is the complete setup? That would help to find the error
Thanks for the fast replies.
But no, I have everything disabled (VPNs/Adblockers).
I kept the automatic configuration on K9.
But it seems now it’s a paid option on Proton:
I don’t think it used to be, what an enshittification
The IMAP option has always been paid and only available through that Proton Bridge, which is why I’ve never switched to Proton and only use it for unimportant stuff that I don’t need to keep. It’ll be a freezing day in hell before I’ll pay for the “privilege” to store MY emails on MY devices 
AFAIK, Proton has always needed that bridge to work with other email clients. Otherwise, you have to play and use their apps. This is the only way to do encryption for data at rest.
If one needs Proton for encrypted at rest emails, best to stay with their apps.
The joys of proprietary, non-FOSS apps 
Proton’s clients are open source. Proton Mail · GitHub
Back end not so, although there may be pieces in these 167 repos they have.