You didn't tell what version of Mac OS X you're using. If you're using a late version of Mac OS X like El Capitan, it may be that you need to update your Wacom drivers. Go to the Wacom website and look for and install a later version.
Yep, sorry, I'll update the original question.
Btw, I'm running Yosemite, but it kept happening even on Maverick.
Have you checked with Wacom tech support?
Nope, I'll try to ask them too.
I'm running Yosemite 10.10.5.