@Stef77 I 've tested with 10m UTP and the reboots stopped. I m searching to buy now a 15m UTP to test also because i need ~13m distance.
(I already updated FW to latest ver 1.10.40 but wasn't fixed anything).
I saw that the reboot problem is only when i connect at 5GHz network and also when transmit power of 5 GHz is on HIGH and Wireless mode is 802.11a/n/ac mixed.
(It never happened with 2.4 with any of settings).
 

  @tplfangr Wow, does that sound like a software problem? Unfortunately, in my case, I need a 20m distance cable for my AP. The strangest thing was that I tested the cable before I ran it and it could power my AP without rebooting. Then I configured the AP locally, laid the cable down and then connected the AP but started the loop reboots. Maybe it's because of the settings you also mentioned. I have 802.11a/n/ac mixed enabled at 5GHz and 2.4GHz. I keep them on low though. I should play a little bit with settings at some point.

  @Stef77 No, I don't think its a SW problem.
I've already tested with a 15m UTP Cat5e simple cable, but even with low settings i mentioned before, sometimes i experienced the reboots. Now, i have tried for the last week, another one from the same manufacturer, 10m UTP Cat5e, which even with HIGH settings, no reboots until now. Either way i need 13m so I'm searching for a better quality cable at 15m, and i have read in some articles that Cat6a (FTP) cables are having POE compliance. I'm thinking buying this one which also is kind expensive (~30€ !!) but I'm not sure that the problem is going to be fixed: "STP6X15MIG" from Panduit (Google it to see).