What happens if I play a player attachment card with the Guarded (enemy or location) keyword, and the enemy (or location) that would guard the card has the "immune to/cannot have player card attachments?

Good question, I am not sure. If this happened while I was playing, I would probably consider it not an eligible target and continue discarding until I found one that could attach.

Another option would be that the immune enemy/location still gets added to the staging area, but since it can't have attachments, the guarded player card doesn't get attached to anything and you... can claim it asap : S

How do you contact Caleb again?

Caleb has ruled in the past that there is no way around the Guarded keyword, so I am pretty sure whatever the ruling would be, it would not end up with an immediately attached Guarded card. Either the Guarded card gets discarded, or you discard the immune enemy and continue looking for an eligible target. I strongly suspect the latter. To send a rules query, though: click the "customer service" link at the bottom of this forum, then click through to 'rules questions.' And let us know what you hear back!

edit: or weird 3rd option, the guarded card is a legal player card attachment for an immune enemy. Move over, Quick Strike...