Well, I've been ignoring it because using sidecut on the place of a basic main-hand attack can do the cut (no pun intended!), but there goes a bump, because this bug kind of harshly slows down a Rogue trying to do basic hits with Dagger Dance. Or a Magic Gunner trying to Akimbo someone down.
This happens/starts happening exactly after a reboot is done, and seems to be only fixed when the game is updated, just to be broken again at random in one reboot. I can confirm that before a reboot I could freely use my main attack and sub-attacks in my hotbar, and after the reboot, my main attack stopped working, then...
I've tried booting up Player Debug after the bug started once again trying to get more information. And did press the main hand, then off-hand (sub-attack) in order, slowly. Only sub-attack was responding the command.
Then I've tried pressing only the main attack repeatedly and a '1' started appearing on the debug. What does that mean?
Also, can more things be added to the debug so it becomes easier to identify the problem? It seems a little crucial. And I think this can't be reproduced in normal circumstances, only on live server after a reboot is done, is what I expect.