Since the last maintenance (11.12.2017) I have quite regularly disconnects with merchants/guild stores. I can still call up the merchant window, but the SEARCH and SUBMIT buttons are not active any more. So I can not do any action any more. Also I can not see my list of items I put on sale, it is emtpy.
When I relog with my character all is fine again. So it really is some kind of disconnect. Everything else still works. For example accessing my bank account and depositing or withdrawing items.