I had some problems with agendav complaining not being able to load certain calendars. After some debugging, I found the source of the problem being the unknown "Romance Standard Time" timezone which I get by accepting Outlook invites in Thunderbird.
According to <
http://msdn.microsoft.com/en-us/library/ms912391%28v=winembedded.11%29.aspx> it is the Windows name for Europe/Brussels, so adding the line
"Romance Standard Time" => "Europe/Brussels"
to the lookupTz function in Timezonemanager.php solves the problem.
My problem is solved, but I thought posting it could be useful for someone else googling on this error...
Best regards,
Maarten