I have the SAME problem with submitting my claim. In fact I am unable to even add my 1st item. Have now been in contact with 'support' for 4 days and little progress other than to tell me their engineers are working on it!
Meanwhile according to airbnb claims must be submitted before the next guest checks in, and yet THEIR 'technical issues' are preventing me from doing just that. I have asked for an alternative means to submit my claim, but not heard anything back.
I have spent a lot of time following their 'troubleshooting' steps, but just basically repeating things on different browsers, computers, clearing caches, history, cookies, etc. things I tried already my self without any success.
Suddenly it's all gone quiet and don't know where I stand, and I am running out of time.