Hi, @scanex (allan nielsen)
(Usual disclaimer: please note that I'm just another Forum user here, I'm NOT an Ubuntu Developer and/or Ubuntu MATE Developer).
Besides the good answers that @tkn and @gordon have already given you in this topic, I suspect (although I may be wrong) there may be an additional issue that is preventing you (and probably other people) from upgrading from Ubuntu MATE 23.04 ("Lunar Lobster") to Ubuntu MATE 23.10 ("Mantic Minotaur"). Specifically, in the Official "Ubuntu Community Hub" / "Ubuntu Community Discourse" - https://discourse.ubuntu.com/ - there is the following "Mantic Minotaur (23.10) Release Status Tracking" topic:
In that topic, one can find, among other useful information, the following excerpt:
" (...) Current blockers for enabling upgrades
That is a link to "Bug #2034986 “some text became unreadable during a distribution upgrade” : Bugs : ubuntu-release-upgrader package : Ubuntu" - https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/2034986 - that relates to a problem that was also mentioned here in the "Ubuntu MATE Community", specifically at the end of the following post by @jmarkus (Jim) in the "Ubuntu MATE 23.10 testing" discussion topic: