bug after patch 1.6

the mouse cursor glitches

Do you mean that the cursor sometimes turns into the standard, Windows cursor? If so, I have the same problem; it seems to be one of the few unpleasant "new features," or rather, one of the bugs in patch 1.6.

In general, try starting the Campaigns again. Everything should work. To make sure that the problem is indeed that you can't finish missions because of the patch, play the very first mission of the first Campaign. It will take no more than 10 minutes. If everything is OK, then play to your heart's content, but start over. Most likely, you will have to replay the mission where you stopped. If that doesn't help, then replay the Campaign. Or, revert to version 1.41.

can you tell me more about mods?

Sure :) A mod is short for modification. In most cases, mods change the appearance of creatures, the interface of the main menu, the game's text, and the balance. There are very simple ones that remove annoying messages (like "the hero can no longer move") and much more complex ones (for example, significantly changing the characteristics of creatures, the skill tree, etc., with the aim of making online battles more balanced and interesting)