JavaScript TypeError when importing an iCal Calendar (orm.catid is undefined & document.adminForm.allDayEvent is undefined)

Hello. I'm unable to add Google Calendar as an iCal source.
It's the No. 1 Reason why my org is unable to switch away from Google iCal Reader until now.

I tried both the Google public iCal link (readonly) as well as the secret iCal link (read and write)

I'm running Firefox 59 on Windows 10.
When trying to add the calendar I get the following JavaScript errors:

TypeError: form.catid is undefined[Learn More] index.php:338:1

	submitbutton https://g5seventyjoomla.de/administrator/index.php:338:1

	onclick https://g5seventyjoomla.de/administrator/index.php:1:99

TypeError: document.adminForm.allDayEvent is undefined[Learn More] editicalJQ.js:485:6

	toggleAllDayEvent https://g5seventyjoomla.de/components/com_jevents/assets/js/editicalJQ.js:485:6

	<anonymous> https://g5seventyjoomla.de/components/com_jevents/assets/js/editicalJQ.js:1108:2

	dispatch https://g5seventyjoomla.de/media/jui/js/jquery.min.js:3:12392

	add/r.handle https://g5seventyjoomla.de/media/jui/js/jquery.min.js:3:9156

	trigger https://g5seventyjoomla.de/media/jui/js/jquery.min.js:3:11571

	a.event.trigger https://g5seventyjoomla.de/media/jui/js/jquery-migrate.min.js:2:8273

	triggerHandler https://g5seventyjoomla.de/media/jui/js/jquery.min.js:3:19064

	ready https://g5seventyjoomla.de/media/jui/js/jquery.min.js:2:30047

	K https://g5seventyjoomla.de/media/jui/js/jquery.min.js:2:30366

Firefox Console Output

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This seems to be a problem with JEvents and/or my Joomla! Instance.
Can you help me?

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Support Info:

PHP Version : 7.0.27

MySQL Version : 5.7.21

Server Information : Linux g5seventyserver 4.4.0-116-generic #140-Ubuntu SMP Mon Feb 12 21:23:04 UTC 2018 x86_64

Fix jQuery? : Yes

Load JEvents Bootstrap CSS? : Yes

Max Input Vars ? : 1000

Club code set? : No  

User Agent : Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:59.0) Gecko/20100101 Firefox/59.0  

Joomla : 3.8.6

component_com_jevents : 3.4.46

component_com_jce : 2.6.26

module_mod_jevents_cal : 3.4.46

module_mod_jevents_custom : 3.4.46

module_mod_jevents_filter : 3.4.46

module_mod_jevents_latest : 3.4.46

module_mod_jevents_legend : 3.4.46

module_mod_jevents_switchview : 3.4.46

plugin_content_jevents : 3.4.46

plugin_finder_jevents : 3.4.46 (not enabled)

plugin_installer_jeventsinstaller : 3.4.46

plugin_search_eventsearch : 3.4.46

plugin_system_gwejson : 3.4.46

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