OK, this is still happening in v2.7.3 (screenshot attached). I have been able to replicate this numerous times now. It seems to occur when I browse through the different level pack/groups, expanding and then contracting different levels in the tree structure on the left. It will work fine to start with, then will temporarily freeze/lock up, and the level select window will get the "...(Not Responding)" message in the title bar. Then it will refresh and become functional again, but with the lemming hand cursor. If I close the level select window and then reopen it, the cursor reverts back to the Windows mouse cursor again. Sometimes (less frequently though) when it stops responding it kills SLX entirely, and I have to completely restart it. This happened when I was testing immediately prior to posting this message.
I am running Windows 10 Pro v22H2 on an Aspire E15 E5-552 (SNID: 54000963676) laptop with 8GB RAM and AMD Radeon R6 on-board graphics, 40GB+ free HD space. I have no other open apps (apart from MS Edge as I had this page open) and all latest drivers/Windows Updates, etc are installed. Since I originally posted this issue I have cleared my system and user profile Temp folders, and have obviously shut down/rebooted numerous times. I do have NordVPN running, but even when I have closed this and all of its associated services (and the same for my AV app), the issue with SLX level select occurs.
It's not a major game-busting issue, and is easy enough to work around, but still is a little frustrating. I will test again on my desktop PC, which is also running the same version of Windows, with a loosely similar spec - for this issue and the panel issue I raised - to see if I can replicate the issues on there, and then let you know.