Thunderbird calendar

It is impossible to edit a calendar item. My calenbar is synchronized with Synology calendar. From there it is synchronized with a number of smartphones, tablets and Windows pc's. Everything performs OK. On my tablets, ohones and pc's I can add and delete items and after automatic synchronization the new data are also onthe the other equipmwent. But editing an appointment performs well on the tablets and phones, but not on the pc's When I edit an appointment on a pc and click "opslaan en sluiten"or "opslaan" the apointment keeps blue, but nothing is changed. Please give me the solution. E-mail works well (via Imap) as contacts, but calendar not. The problem is not the access into the calendar on the synology server, sine I can add and delete appointments. Only editing of an existing appointmen fails.