I tried many times (like 20-30) to post almost what has now become https://www.phpbb.com/community/viewtopic.php?p=16077268#p16077268 - however, that is not the content I wanted to post. There is an asterisk which is in two lines later explained. I wanted to insert the brackets and the text in them at the position where the asterisk is now.

The original content I wanted to submit was: 

Ask him for precise steps, preferably with screenshots. If it works for all others (or at least for you) then his environment is the problem. It could be: [list]

[*] a browser extension/addon he's not aware of which effectively deletes cookies/caches

[*] he uses a private tab without telling you so

[*] he's not even speaking of phpBB's checkbox to be ticked, but may instead mean some browser feature

[*] his understanding is plain wrong (i.e. by setting this in one environment should also show effect in other environments, like ticking the checkbox on the phone but later wanting to use his PC)[/list] If you can monitor him (f.e. via TeamViewer) or he can record a video as a proof it would be a start. 

Then I either click the button "Preview" or "Submit" and it always leads to a Cloudflare check:

Clicking its checkbox then returns me to an empty editor: neither do I have a preview, nor do I have the text I've typed. All is gone. I can reconstruct this 100% with the text given above, so effectively I'm unable to ever post that very content.

I tried portions of the text and it turned out that the brackets with the TeamViewer part seems to trigger Cloudflare for no logical reason. What's going on? Who affects this too, so others users might have given up to post?

I'm not on a VPN. Using Vivaldi 5.6 as browser. I made other posts recently - no problem there (yet).