Hi, and thank you all for the feedback.
The computer runs Win 10 with the latest updates, on a Ryzen 5 2600X CPU and 16 GB of RAM, which also has a GeForce GTX 1060 6 GB video card - the hardware is stable and no other programs ever hang or crash.
Windows Defender is configured as advised on the forums, on all the relevant folders.
The same machine, without any tweaks to Windows Defender, ran MC25 for a year with zero issues, rock stable and reliable - as soon as I upgraded to MC26, issues began and persist to this day. I have also made sure to uninstall MC25, its database, created a new library in MC26, etc. - to no avail.
To clarify - the issue is not crashes, but system hangs. They are frequent and super-annoying - MC will hang and become unresponsive while the PC is doing nothing much at all, and it either takes it minutes to recover, or it doesn't and needs to be force-closed. If the issue were always the same, it would be easy to post the logs here and maybe ask for advice - problem is, it's always something else, and the frequency is totally random. If I were to guess, it is probably an issue with RAM management in MC26, but I am not an expert; all I can say for certain is that MC25 runs with zero issues on the same PC that MC26 seems to have trouble with.
I have also had the same issues with 26 on a different (much less powerful) mini-PC that I use as a HTPC, to the point where I rolled back to 25 on that one, while trying to figure out what is happening with MC26 on my main machine.
I have been using MC since version 12 or so, and more often than not this problem has happened when upgrading - the new version is terribly unstable for the longest time, until things settle down and then it works great. I live in hope that the same will be true for MC26 soon - and thanks again for the responses, much appreciated.