I've tested this and it's not a hotbar problem, it's a problem with what item is in your hands, if you have a canteen/PET in hands it will fill it up, if you don't have anything in hands then the character will drink and if you have any other item in hands you will get that error "Item x can not be filled up" or w/e.

Hello NiMeRi75 and thank you for the report.
The issue has been confirmed and scheduled for a fix.
Regards,
Geez

I noticed something similar, and perhaps there is a link.

I noticed that whenever I would get into a truck and come out, my hotbar would do stupid things like this with a water pump. It would act as if I had a weapon in my hands (No animation of it on character), it also would not allow me to change weapons in the hotbar, and it would block me from drinking with "You cannot fill X"... Even though my hands were empty.

I was able to clear it by relogging.
I was then able to reproduce it by entering and exiting a truck.
Cleared by relogging again.

I tried to reproduce this again the same evening and could not. But I did not spend a lot of time on it. I was unsure if I entered the truck with an item in hand etc... But I have a feeling there may be something in common with the issues.

So it might be, if you get in a Truck while holding a Weapon, it messes the status of the Weapon or inverses it and when getting out you don't hold the Weapon, but the System thinks you do, because the parameter is inverted?!

Just thinking.

Hello NiMeRi75.
The issue has been fixed internally and should be fixed on Steam soon.
Regards,
Geez