Encore Blu-Ray DL project: Not enough space on media... despite having 13GB free...

Please help...

I worked years to put my study project/film on a proper DVD to share the finished work with everyone who was involved...

The DVD (a DL almost 8GB DVD) works perfectly fine, Encore find's it's own layer break and voilà: ISO and then burn to Disk, plays beautifully. 

Now I painstakingly recreated this Disk with many Menus and many smaller videos and one long movie for a Blu-Ray project. 

After trying to create the ISO I always get "Code 25 not enough space on media".

This does not make sense. The Blu-Ray is supposed to be a DL (almost) 50GB one and all timelines could fit on just one layer: One video and audio timeline combined are about 22GB, then several small video timelines with 200MB - 6GB. All in all there is 34GB needed. ENough space for a DL, right? The big timeline has enough chapters so I don't see a reason not to find a layer break point...

Can someone please help me troubleshooting, I'm going insane... I'd like to know how to see that log file after the error but I just can't find out how...

I'm on an iMac with 10.12.6 with the latest and last Encore version available (back when I could download it with my CC subscription, so sadly I can't download it to another computer or even Windows...).