iCal import does not work

[ClemensRobbenhaar]

When trying to import an ical file into a calendar, this produces an error message:

There was an error while importing the calendar data. Please try again or consult the logs!

Example iCal import:
local_test.ics.txt (added *.txt suffix to be able to upload the file)

Steps to reproduce:

• from the main calendar page, go to any calendar (at least "Other Events" should be available)
• on that calendar page, click the "import .ics file" button
• in the dialog window that opens, upload the iCal file via ".ics file to be imported" and click "Begin import"

Instead of importing events the error message appears.

There is nothing in the normal logs, but the log.xml in the data/status/moccacalendar/import/<calendar%20name>/log.xml

       <detailMessage>Invalid TimeZone ID: []</detailMessage>
      <stackTrace>
        <trace>org.apache.commons.lang3.Validate.notBlank(Validate.java:782)</trace>
        <trace>wiki:xwiki//net.fortuna.ical4j.model.TimeZoneRegistryImpl.getTimeZone(TimeZoneRegistryImpl.java:180)</trace>
        <trace>wiki:xwiki//org.xwiki.fullcalendar.internal.CalendarReader.getTimeZone(CalendarReader.java:98)</trace>
        <trace>wiki:xwiki//org.xwiki.fullcalendar.internal.DefaultFullCalendarManager.getICalEventsFromFile(DefaultFullCalendarManager.java:132)</trace>
        <trace>wiki:xwiki//org.xwiki.contrib.moccacalendar.internal.importJob.ImportJob.runInternal(ImportJob.java:97)</trace>
        <trace>org.xwiki.job.AbstractJob.runInContext(AbstractJob.java:246)</trace>
        <trace>org.xwiki.job.AbstractJob.run(AbstractJob.java:223)</trace>
        <trace>java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)</trace>
        <trace>java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)</trace>
        <trace>java.base/java.lang.Thread.run(Thread.java:829)</trace>
      </stackTrace>

This happens independently of being a time zone send in the wiki administation or keeping the "default timezone"
Calendar version is 2.15