Strange fast ringing from HT818


I recently setup a new HT818 in front of a Mitel PBX. I’m running firmware version I have 5 extensions, not in a hunt group, using PJSIP to an offisite VOIP server. The unit rings in and out just fine for a day or two and then randomly goes to a fast, almost double ring when dialing inbound from the caller side and doesn’t ring anything on the PBX. Calling out still works as normal. A hard restart fixes the issue and it starts working as intended again. The reboot button on the GUI locks the unit up and I’m unable to log back into it. Any ideas?