Calendar: ics import not working

Nextcloud version: 22.2.3 (stable channel)
Operating system and version: Raspbian 10 Buster
Apache or nginx version: 2.4.38
PHP version: 7.3.31
Calendar : 3.0.5

If I try to import an .ics file that has more than one vcalendar entry, the import fails with the messages No valid file found, import aborted. and myfile.ics couldnt be analyzed (translated).
With only one entry the import succeeds (I copied several single vcalendar entries to separate files - all worked).
The file has ~140 entries and is succesfully validated with https://icalendar.org/validator.html and https://mozilla-comm.github.io/ical.js/validator.html

First I thought this may be related to Calendar - Unable to import an Nextcloud's calendar export (import doesnt work with firefox), but the same error happens also with chromium.

The next hint was the firefox error console: it shows me the following exception:

TypeError: t.toUpperCase is not a function
    b index.esm.js:106
    K index.esm.js:658
    Tt index.esm.js:984
    fromICALJs index.esm.js:658
    fromICALJs index.esm.js:984
    _createCalendarComponent index.esm.js:1217
    parse index.esm.js:1217
    n SettingsImportSection.vue:211
    u runtime.js:63
    _invoke runtime.js:294
    v runtime.js:119
    gp calendar-main.js:2
    o calendar-main.js:2
    promise callback*gp calendar-main.js:2
    o calendar-main.js:2
    Tp calendar-main.js:2
    Tp calendar-main.js:2
    processFiles SettingsImportSection.vue:161
    VueJS 3