Hi,
Well I use the same procedure - and I downgraded to 2.9.4 and it worked again.
What I use is a template of a previous page to get consistent look-and-feel for the site.
The template consists of 4 thumbnails in a row and expanded to several rows. So there is previous content that I exchange if that is of concern for the issue.
The code looks like this:
<div id="wrap">
<div class="box">
<div class="innerContent"><a href="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/kringarbete11.jpg" target="_blank" type="image/jpeg" class="jcepopup noicon" data-mediabox="1" data-mediabox-group="momo-kring1" data-mediabox-title="Amfiteatern tar form!" data-mediabox-caption="Foto: Zacharias Östman" data-mediabox-src="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/kringarbete11.jpg" data-mediabox-type="image/jpeg"><img src="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/thumbnails/thumb_kringarbete11.jpg" alt="kringarbete11" width="280" height="280" /></a></div>
</div>
<div class="box">
<div class="innerContent"><a href="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/kringarbete13.jpg" target="_blank" type="image/jpeg" class="jcepopup noicon" data-mediabox="1" data-mediabox-group="momo-kring1" data-mediabox-title="Zacke och Anders Bygger" data-mediabox-caption="Foto: Zacharias Östman" data-mediabox-src="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/kringarbete13.jpg" data-mediabox-type="image/jpeg"><img src="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/thumbnails/thumb_kringarbete13.jpg" alt="kringarbete13" width="280" height="280" /></a></div>
</div>
<div class="box">
<div class="innerContent"><a href="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/kringarbete12.jpg" target="_blank" type="image/jpeg" class="jcepopup noicon" data-mediabox="1" data-mediabox-group="momo-kring1" data-mediabox-title="Kostymbossen Mikael Tjäder" data-mediabox-caption="Foto: Zacharias Östman" data-mediabox-src="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/kringarbete12.jpg" data-mediabox-type="image/jpeg"><img src="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/thumbnails/thumb_kringarbete12.jpg" alt="kringarbete12" width="280" height="280" /></a></div>
</div>
<div class="box">
<div class="innerContent"><a href="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/kringarbete10.jpg" target="_blank" type="image/jpeg" class="jcepopup noicon" data-mediabox="1" data-mediabox-group="momo-kring1" data-mediabox-title="Välbehövlig paus för Sussi och Anton" data-mediabox-caption="Foto: Zacharias Östman" data-mediabox-src="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/kringarbete10.jpg" data-mediabox-type="image/jpeg"><img src="https://www.joomlacontenteditor.net/images/momo/img/kringarbete/thumbnails/thumb_kringarbete10.jpg" alt="kringarbete10" width="280" height="280" /></a></div>
</div>
</div>
One other issue I had with 2.9.4 was that the " data-mediabox-src=" didn't get updated so I had to open the procedure again to view the select popup and save and then it got updated.
Regards
Dennis