BUT if u want to test getting rid of the message do this

edit ~/MagicMirror/modules/default/calendar/calendarutils.js

	if (event.start.tz.includes(" ")) {  // this is line 37 , not changed
	        let tz = null                // first replacement line
		// if the tz is the MS customized TZ
		if(event.start.tz ==='Customized Time Zone')
			// use the locally set tz
			tz=moment.tz.guess()
		else
			// use the lookup table to get the IANA name as moment and date don't know MS timezones
			tz = CalendarUtils.getIanaTZFromMS(event.start.tz);  // last replacement line
			Log.debug("corrected TZ=" + tz);      // line not to be changed

replace the lines BETWEEN line 37 and the last line (also not changed)
with first replacement line thru last replacement line
so it looks like that

BEGIN:VEVENT
UID: ....
SUMMARY:Verpakkingen
DTSTART;TZID=Customized Time Zone:20210107T080000
DTEND;TZID=Customized Time Zone:20210107T090000
CLASS:PUBLIC
PRIORITY:5
DTSTAMP:20211030T090840Z
TRANSP:OPAQUE
STATUS:CONFIRMED
SEQUENCE:0
LOCATION:
X-MICROSOFT-CDO-APPT-SEQUENCE:0
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
X-MICROSOFT-CDO-INTENDEDSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:FALSE
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-CDO-INSTTYPE:0
X-MICROSOFT-DONOTFORWARDMEETING:FALSE
X-MICROSOFT-DISALLOW-COUNTER:FALSE
END:VEVENT

Thanks in advance

can u show me a cal entry from the ics file (use the url to download the file, and then u can edit it, as it is just text)

a cal event starts with BEGIN:VEVENT
and ends with END:VEVENT