When AC power is not used (for example, when you use PoE or 12 VDC to power the access point), you should cover the unused connector. A cover is available from Remke (see http://www.remke.com for part number 75-0086).
A little late reply but I hope this could come handy for others having the same problems.
I ran into this issue as well. The best part is that Remke.com has a minimum order requirement.
"Minimum order amount is: $100.00"
Its pretty frustrating that the AIR-CAP1552 units come with lots of accessores like a grounding lug and COAX-SEAL compound to weather proof the unit but it doesnt come with a power cover! Its crazy considering how many people will power these units with PoE these days.
Just for refrence my unit was an AIR-CAP1552E-A-K9 so it looks like this is an issue with the 1520 and 1550 units. Definantly something to watch out for when ordering these.
Also for refrence, it may seam trivial but when you see the power connector you will get it. Its number "1" in the picture below. Cover must be ordered seperatly!!
"When AC power is not used (for example, when you use PoE or 12 VDC to power the access point), you should cover the unused connector. A cover is included with the power injector and is also available from Remke (visit http://www.remke.com and search for part number 75-0086)."
So, this cover is included with AIR-PWRINJ1500-2=.