Errors in Log: TimeZoneUtil.php

Hi

My log (Admin>Logging) gets spammed with the errors: Every 30 min I get about 35 times:

Error PHP Uninitialized string offset: 0 at /opt/nextcloud/3rdparty/sabre/vobject/lib/TimeZoneUtil.php#138 2017-06-22T14:43:33+0200

Until now everything works as expected, but wouldn’t notice if another error shows up.
Any idea how to solve this?

Thanks

cat /etc/timezone

yields: Europe/Zurich

phpinfo shows date.timezone is set to Europe/Zurich

Nextcloud version: 12.0.0
Operating system and version: Debian stretch stable
Apache or nginx version: nginx 1.10.3-1
PHP version: 7.0.19-1

Is this the first time you’ve seen this error?:
yes.

The output of your Nextcloud log in Admin > Logging:
35 times every 30 minutes and after creating a calendar entry:

Error	PHP	Uninitialized string offset: 0 at /opt/nextcloud/3rdparty/sabre/vobject/lib/TimeZoneUtil.php#138	2017-06-22T14:43:33+0200
Error	PHP	Uninitialized string offset: 0 at /opt/nextcloud/3rdparty/sabre/vobject/lib/TimeZoneUtil.php#138	2017-06-22T14:43:33+0200

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

https://pastebin.com/8G85yjt1

On your admin page, is your code integrity check showing errors? If not, please report directly to the bug tracker on https://github.com/nextcloud/server/issues

No, it says all examinations passed.

While gathering some information for the issue on github I realized I imported two calendars. One of them must have had awrong TZID. As soon as I deleted the calendar (and recreated it manually…) the log was ok.

I exported both calendars from outlook.com via MS Outlook 2010. As only one calander with few events went wrong I won’t dig deeper into this.