My glowforge laser tube started filling up with bubbles a few days ago but everything else was working as it should (no loss of power, not overheating). I didn’t see any visible signs of leaks so I kept using the machine.
The bubbles would not go away.
After reading through a few threads here in the forum, the possible causes for this problem were never revealed by GF support team and the threads were closed. The community however, usually attributed this to possible leaks, cracked laser tube or low coolant levels. Nobody seemed to really know what the problem was either.
This morning I decided to inspect the machine again and found liquid in the bed area only. I figured the leak had to be coming from the laser arm area somewhere.
Sure enough.
Behind the right hand side of the laser tube where the coolant hose loops back behind and underneath the laser tube, there is a bearing. That bearing had been chafing against the hose for who knows how long. (long enough to cut through the hose)
That’s a super tight area and even when comparing that to other glowforges that I have access to. All machines have minimal space between the hose and the bearing.
I’m no saying that every Glowforge machine will have the same issue but I will start checking that area regularly from now on. I don’t know if my glowforge came like this from the factory or if the hose sagged just a little bit overtime that it started chafing against the bearing.
As a precaution, you should check that area in your glowforge too 
hoping my problems help you prevent yours.
(I just opened a Support Case but haven’t heard back yet, I will update this as soon as I do)
