[ Thanks to PmaBox for this link.
]
“a) Basically you immediately find this bug in all
calendars. MODIFICATION_FAILED which means that after you added the
calendar and it shows all the events, if you try to add, change or
delete an event you get bumped with this error message and the
calendar turns to read-only mode. Therefore renders the calendars
useless in the client.“b) Another bug is related to https hosted calendars. In these
cases we need to add a (signed or not) security certificate from
the server. Thunderbird keeps forgeting or does not store the
certificate, so each time you start Thunderbird you get this
annoying and time consuming certificate pop-op. After you accept
the certificate you get the error message.”