Calendars are disappearing after adding external URL

Dear community,

I have a problem with JEvents and I do not know if anyone can confirm this or if it is a known bug.

What I want to do?
I want to show a calender in JEvents imported from ICal in order to allow simple administration of the events in various end devices.

Steps to reproduce:

• Install a plain Joomla installation somewhere without any example data.
  
• Install JEvents
  
• Go to the list of calendars (needs to confirm default settings) in the backend
  
• One default calendar is shown there
  
• Add a external URL-based calendar with a calendar URL https://nextcloud.christian-wolf.click/nextcloud/remote.php/dav/public-calendars/jW7s95LXXyy7BLR4?export
  
• Reload site or navigate around and return to list of calendars
  

Expected behavior:
First only one calendar is shown and can be accessed.
Then the calendar is added. When returning to the list of calendars, both calendars should be shown and accessible.

Real behavior:
The first calender is successfully shown.
After addition all calendars get hidden. No one (not even the static, first one) is shown.

I found this problem on a production server, I do not want to play with. Thus I tried am minimal setup and had the very same effect.

Here is my settings (test system):
<code>
PHP Version : 7.2.11
MySQL Version : 5.5.5-10.1.36-MariaDB
Server Information : Linux server-hh 4.18.16-arch1-1-ARCH #1 SMP PREEMPT Sat Oct 20 22:06:45 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 (X11; Linux x86_64; rv:62.0) Gecko/20100101 Firefox/62.0
Joomla : 3.9.0
component_com_jevents : 3.4.48
module_mod_jevents_cal : 3.4.48
module_mod_jevents_custom : 3.4.48
module_mod_jevents_filter : 3.4.48
module_mod_jevents_latest : 3.4.48
module_mod_jevents_legend : 3.4.48
module_mod_jevents_switchview : 3.4.48
plugin_content_jevents : 3.4.48
plugin_finder_jevents : 3.4.48 (not enabled)
plugin_installer_jeventsinstaller : 3.4.48
plugin_search_eventsearch : 3.4.48
plugin_system_gwejson : 3.4.44
</code>

Same result here:
<code>
PHP Version : 5.6.38-0+deb8u1
MySQL Version : 5.5.60-0+deb8u1
Server Information : Linux vps1 2.6.32-48-pve #1 SMP Sat Jan 6 13:23:02 CET 2018 x86_64
Fix jQuery? : Yes
Load JEvents Bootstrap CSS? : Yes
Max Input Vars ? : 1000
Club code set? : No
User Agent : Mozilla/5.0 (X11; Linux x86_64; rv:62.0) Gecko/20100101 Firefox/62.0
Joomla : 3.9.0
component_com_jevents : 3.4.48
component_com_jce : 2.6.12
module_mod_jevents_cal : 3.4.48
module_mod_jevents_custom : 3.4.48
module_mod_jevents_filter : 3.4.48
module_mod_jevents_latest : 3.4.48
module_mod_jevents_legend : 3.4.48
module_mod_jevents_switchview : 3.4.48
plugin_content_jevents : 3.4.48
plugin_finder_jevents : 3.4.48 (not enabled)
plugin_installer_jeventsinstaller : 3.4.48
plugin_search_eventsearch : 3.4.48
plugin_system_gwejson : 3.4.44 (not enabled)
</code>

I just saw that the settings contains something with the text `gwejson`. During installation there was a message that something went wrong with the installation there. Unfortunately I do no longer remind the text exactly but some installation file was not found as far as I remember. This could be related to the main problem but not necessarily.

Thanks for your support.
Christian