ACF pro + ACF Theme code Pro – but a completely blank page

Hello and HAPPY EASTER!!!!

I hope somebody may help me, though it’s holiday time.

I’m in trouble:
I bought ACF pro and ACF Theme code pro to build a custom single post template,
after a successful try with the free versions.
(Though I’m new and not firm in coding:
– I’ve successfully created a custom-single post.php folder via FTP
– Successfully modified the code
– The ACF fields were displayed correctly in the post template
– I was able to save and update the post content
– the content of the ACF fields was correctly shown in the frontend
– and the frontend-view of my side was complete (including Side-title, tagline, menu, post content, comments and else)

After this promising start, I tried the same procedure with the pro versions of both plugins, as I need the “flexible Content” function for a multiple column layout.
I started by creating EXACTLY the same layout like shown in the Video in the flexible Content section of the documentation.

But unfortunately, literally NOTHING seems to work:
1. (the “flexible content” group is shown in the post template and I’m able to put some content in the various fields)
– BUT: as soon as I try to save and publish (or update) the post, ALL INSERTED CONTENT IS VANISHED (Tried this about ten times but nothing helped).
2. Though I’ve modified the single-custom-post.php in the same way as the first time (just replaced the code for the field group by the new code for the flexible content that “ACF Theme Code Pro” provided), my frontend-view is COMPLETELY BLANK. (COMPLETE BLANK, meaning NO SIDE TITLE, NO TAGLINE, NO CONTENT etc., etc.. ABSOLUTELY NOTHING!) (please have a look here)

I’m trying for hours to find a logical explanation, as I think if the error is in the ACF code, at minimum my side title, tagline, and so on should be visible. I am at my wit’s end.

I would be INEXPRESSIBLE grateful for some quick help as I’ve intended to benefit from the free Easter days to manage my custom post problem. I will provide Admin and login data if someone consents to help me, out of my WP-nightmare.

With thanks in advance-
Reca

Additional information:
-I’m using the latest versions of WP, ACF and Theme Code pro.
-I’m using the German ACF version
– My theme is “Coral light pro” by “Coralthemes”
-I’ve deactivated the free version (activating it again doesn’t solve my problems
-As my first try (test with free versions) still has a correct view, I don’t think another plugin causes a conflict or else.