Comment # 5 on bug 944768 from Marcel Waldvogel

This problem still exists and changing to an IPv4-only hostname didn't solve
the problem. Deleting one of the readonly calendars and adding it again fixed
it for that particular container. I did not have the patience to go through the
dozens of calendars to which I am subscribed.

Instead, I quit Evolution and killed all evolution* processes (the killing is
important); after a relaunch of Evolution, the data sources were writable
again.

I am not sure whether the IPv4 stuff is necessary. If it happens again, I'll
just try killing and relaunching. For now, I believe that it was just a
temporary network downtime (not too unexpected for a Laptop being carried
around) which caused the readonly flag to be set.