I don't have a J! 3.4 stable site yet so I tried yesterday with J! 3.4. RC2. And there I got 404 error in both popups.
Today I'm busy but think on weekend I will setup a J! 3.4 stable site and debug/analyse this part. If it works there I must change my question from "What's wrong?" to "Why does it work now?"
If it was a Joomla bug in RC2 ok, if it's accident we need a solution to prevent this in the future.
Oh my, as if we hadn't already enough other tasks...
Edit: For statistics: I updated from J! 3.3.6 to 3.4 RC2. If I remember right the 3.3.6 was an update from 2.5.
on the same fresh Joomla 3.4 stable I tested with JEM 3.0.7 before, I uninstalled this JEM (included with cleanup yes)
and installed JEM 2.1.3 (with the changed script)
I am on official protostar template.
I imported with csv some events, categories, venues and catevents.
I can choose any venue with the modal window in frontend.
Linux Server #1 SMP Debian 3.2.65-1+deb7u1 x86_64
PHP-Interface für den Webserver cgi-fcgi
Joomla!-Version Joomla! 3.4.0 Stable [ Ember ] 24-February-2015 23:00 GMT
Joomla!-Plattform-Version Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
Browsererkennung Mozilla/5.0 (Windows NT 6.1; WOW64; rv:35.0) Gecko/20100101 Firefox/35.0
Testing JEM 2.3.0 beta3 local on clean latest Joomla (3.9.11) Protostar php7.3.8 (no mails)