Error when accepting a meeting invitation in Thunderbird

Hi,
When I share a calendar with Domain Users but do not give them Edit rights to the Calendar and after I invite them to a meeting and they accept the invitation they get the following message in Thunderbird.

Any suggestions?
Best regards,
Ana

@anuschka , could you provide some log files? owncloud.log apache error and access log?
which version from oC are you using?