Yello Triangle at Calendar and Contacts

Since the last 2 or 3 updates there are after some hours yellow triangles at calendar accounts (google, synology …) and at contact accounts (synology). They haven´t been there before. when i restart, some triangles disappear, but they are back after a several time. The accounts are reachable.

Do you have any Idea?

You will find errors for this in Menu > Operations > Log tab. Look for Google Calendar and CalDAV and CardDAV (for synology) errors.