Tried to re-add HotDesking devices, got an error "Object reference not set to an instance of an object"

[JanFilipowski]

When I had to temporary delete my phones from HotDesking in order to test them on different PBX and then I tried to add them again, I gon an error "Object reference not set to an instance of an object" right after I selected model and MAC address. Befor they all were working like a charm. I've used a different browser, waited for a few days - but it seems that it is an issue related to PBX, which inevitably I have to fix myself. Thank you in advance for help.

 

[JanFilipowski]

After I have written this I realised that it is not only hotdesking issue, but I also cannot assign traditional extensions to my phone in the "telephones" tab. However I can add new extensions.