"Cannot Use that Class" message????

[Mhaeric]

I have had issues with gear sets not equipping if I have removed something from the armoury chest

This is a design choice, not a bug. I suspect it would be far more resource intensive to search your inventory for an set item rather than the armoury chest. This is an incredibly minor inconvenience when it (rarely) happens, so I don't support changing it unless it can be done without impacting server load.

Same here. If one item of a created set is missing it won't equip anything of said set. I find the whole set system quite bad. I mean items are not listed as being part of a set for instance. So it's very easy to sell/store an item somewhere without realizing it belongs to a set. And making that set unequipable.

The only time a set won't be equipped is when you are missing the primary weapon, which should be obvious is not a bug. Any other missing piece from a set will result in a partially equipped set and a message indicating such. Heck, even if one of the pieces was dyed a different colour, it still equips it and lets you know it equipped a different colour item.

That said, to help out in the rare cases someone is stupid enough to sell their primary weapon, a better design choice would be able to switch items out of a set via the 'Display Set' menu by right-clicking on the item in the list. Right-click would give the option to 'Replace Item,' clicking on it would bring up a window similar to the 'Hand Over' window with a slot you can drag your new chosen item into as a replacement.

To the OP: That sounds like a bug to me, since it resolved itself upon re-logging. If you do submit a bug report, try to include as much detail as possible. E.g. which class, which abilities you tried to use, if you had changed classes or it just started not working, what actions you had done right before it stopped working, etc.