Piotr Zarecki 

To be precise: you do not mean the "left" ALT key, but the right one, the AltGr (or CTRL+ALT) key? If so, please check in your shortcuts, if there is something else assigned to CTRL

ALT+Z. I am not using the Polish programmers keyboard (QWERTY), but the Polish 214 on QWERTZ and have no such problems. TBH, I have never had problems with Polish letters with the Polish 214 layout, as the PL letters are hardcoded there.

Yes, I mean the rigt ALT key, the Alt Gr key.

Yes, I am using the Polish programmers keyboard (QWERTY), but I have never had problems with Polish letters and in Trados since 15 years.

The only similary shortcut is Ctrl+Z, but removing the keyboard shortcut "Undo" (Ctrl+Z) in Trados does not help.

Nevertheless, many thanks for your answer Jerzy.

Yes, I mean the rigt ALT key, the Alt Gr key.

Yes, I am using the Polish programmers keyboard (QWERTY), but I have never had problems with Polish letters and in Trados since 15 years.

The only similary shortcut is Ctrl+Z, but removing the keyboard shortcut "Undo" (Ctrl+Z) in Trados does not help.

Nevertheless, many thanks for your answer Jerzy.

Piotr Zarecki 

I have checked in my installation and CTRL+ALT+Z (ż) is not used. Also CTRL+ALT+SHIFT+Z (Ż) seems not to be used. As the behavior you describe is quite strange, please try to reset window layout (via View menu). Very often this solves problems with the graphical interface of Studio. Otherwise I have no clue what can be causing this.