From which version of phplist? From the screenshot it looks like a problem with quoted-printable encoding. Do you have any lines in your config file to do with that?
Sorry for lasting long! Our previous version was 3.2.1. So we had a little time i tried to downgrade to this version without doing anything with the database and phplist worked fine.
The “now” situation is an 3.2.1 version - files - and the last version database. So the problem is one of the phplist files, i think.
Your problem looks to be the same as described in this other topic BUG / Hotmail / phplist 3.2.3 caused by an upgrade to the phpmailer package that is used by phplist.
You can copy the phpmailer version that is working for you elsewhere and tell phplist to always use that, so you could then repeat your upgrade of phplist, BUG / Hotmail / phplist 3.2.3