Now I can positively confirm, that the problem was given by a Nouveau driver. According to the receipt in bug # 763052 (#19), I removed the Nouveau driver, restarted for sure, allowed the Nvidia propriety driver (to run Ubiquity in 3D), and everything is going fine now.
Anyway, the bug still persisted until yesterday, and if there is some priority to make the Nouveau driver run with this card, I would be really happy to help as a testing case. Don't hesitate to contact me on o.petris$volny.cz (with some patience, it takes sometimes some days for me to check mails.
Now I can positively confirm, that the problem was given by a Nouveau driver. According to the receipt in bug # 763052 (#19), I removed the Nouveau driver, restarted for sure, allowed the Nvidia propriety driver (to run Ubiquity in 3D), and everything is going fine now.
Anyway, the bug still persisted until yesterday, and if there is some priority to make the Nouveau driver run with this card, I would be really happy to help as a testing case. Don't hesitate to contact me on o.petris$volny.cz (with some patience, it takes sometimes some days for me to check mails.