calendar temporarily not available

[baritone]

I am using a dual boot system with Win7 / Linux Mint Mate 18
with Fossamail 64bit / using lightning in the latest versions and German language packs. I am very happy with Fossamail except:

For several weeks now - unfortunately I do not know since when exactly - most of my calendars have a yellow triangle next to the name with the above mentioned error message. The events do not show in the calendar although I found, that at least some alerts do work even if I cannot see the relevant event.

This does not happen in Linux. There the calendars fill even faster than I can watch.

The links are exactly the same. The spelling is identical.

Most of the calendars are hosted on an OwnCloud installation in connection with my e-mail account at openmailbox.org. In some forums it was mentioned that lightning might only accept calendars with proper UTF-8 coding or refuse to load the files. I have no idea nor any influence on how owncloud encodes the calendars.
Of course all these forums were refering to Thunderbird, not Fossamail...

And of course, in the linux-version everything is fine. Any Ideas?
Thanks. Claus