Thank you guys! I tried again a new password and now it has been working for two days. So apparently the special characters were indeed the problem. Very strange. Works with both host names.

@michelbdec Please be more precise. Ubuntu Touch is NOT Ubuntu. Apps are using .click files. There's no Ubuntu software in Ubuntu Touch, but an open store: https://open-store.io/ in which you'll find "Dekko 2", which is currently the only email client available for Ubuntu Touch. (and works pretty well) Btw, you can install some other Linux software using a Libertine container, but they might not work as expected, especially since they aren't design for a smartphone screen.