Modest is at version 3.90.7-13.5 (apparently the same in testing and devel), libtinymail-camel-1.0-0 is at 1.1.92+1cssu7 (apparently the same in testing and devel, and recommended to fix some ssl/tls issues)

After some fiddling I managed to get the connection work like this:
- Incoming mail server (IMAP) with TLS using port 143
- Outgoing mail server (SMTP) with TLS using port 587

According to my mail provider this means that the connection is actually STARTTLS and not TLS/SSL because of the ports I'm using.

Using the ports for full TLS/SSL encryption I get errors, it does not work with either TLS or SSL setting in Modest's account preferences.

I wonder what that means. Why can it start unencrypted and then switch to an encrypted connection, but can not start an encrypted connection straight away? This is how my desktop clients are set up for the same provider.
I can see on the linked page that "The connection to Posteo is always encrypted (STARTTLS, TLS or SSL)" but it makes me wonder nevertheless.
Can I check this somehow? Full error messages?

Does Modest have settings/config/log files somewhere?
Could not 'find' anything.

On a side note, is there another good mail client?