I found this information.
For some reason it didn't quite work out according to the guide for me, I will explain below.
First the information I found about this problem.
You get this in the error console after activating debugging logs
( activate logging: Click tools -> options -> Advanced -> Config Editor, then search for these keys " calendar.debug.log " set to true and " calendar.debug.verbose.log " set to true )
This is what it looks like in the error console:
[calGoogleCalendar] Adding item https://www.google.com/calendar/feeds/username%40gmail.com/private/full to queue
This is what I found about this error message.
Search for thise key network.http.spdy.enabled and toggle it to false by double-clicking it - this disables the spdy protocol.
Restart Thunderbird and check if the calendar function is working now.
If it does we can try another approach to this problem.
Try this, find the same key again: network.http.spdy.enabled and revert it to true, then locate the key " network.http.keep-alive " and change it's value from false to true
Restart Thunderbird and test if the calendar works now. If it does you are done.
If you like me already have the " network.http.keep-alive " key at true, try to set it to false instead.
Restart Thunderbird and check if calendar functions work now. If they do you are all set.
There might be other keys to test to that start with " network.http. "
Also, all keys above inside hyphens, " ". you are to exclude the hyphens when searching. Like " network.http.keep-alive " is really only network.http.keep-alive
Source of information: Here!