ToME: the Tales of Maj'Eyal

I was walking in the old forest when I meet a loremaster, first time. He looks slow, then he stucks. I dug in front of him but doesn't help. I explored the map clearing it from monsters, thinking that perhaps the portal was blocked. In fact there was a trap over the portal. I went back searching for the loremaster but I couldn't find him. Then when I was walking to the exit the window of the completed quest appears, but it was so fast that I clicked somewhere and the window was already gone. I discovered that I took the spit poison power, which draws equilibrium. I was an archmage so without a means to lower equilibrium. And the spit poison shows with the right-click popup menu so it's easy to click in it. I had to quit the game without saving and reload it.

I've had this happen to me before. It wasn't with an archmage, but on several occasions I've been clicking like crazy in the middle of a tense fight, and then went, "Wait, where'd my escort go.. oh, I seem to have mysteriously acquired some chant.."

Probably, he targeted a creature that he could not reach. This would cause him to seem to freeze. In fact, he is waiting. Until you cleared a path to that creature (probably by killing stuff).
After that, he would have happily and naively continued onwards to the portal, and with some luck, made it there in your case.