I was having much the same issues. Mc_Etlam has posted a "debug" build at https://github.com/mrkite/minutor/wiki/Nighlty-Builds that addresses some of those issues, and seemed to fix my problems. This is not the official release, but well worth a try. Mc_Etlam would probably appreciate it if you post your results here.
Since I see now 2 links on the Nightly-Builds page (and since for me the 'previous' nightly build was work without much problems (only when I zoomed out to max zoom Minutor sometimes crashes)), can you put a date-tag to the links (or in the links) so we can see which is the latest?
Currently I can't see if the debug-build or the inofficial-build is the latest.
Thanx in advance (for keeping this cool tool updated).
Been running into this problem as well. I was able to open an older map from another version of Minecraft, but when I updated that map - the same thing is now happening. Minutor just crashes when trying to load a map from 1.15.1 Java Minecraft