2!

So it's correct... my final suggestion would be strictly to debug your codes, if you could do this... I.e. you need to put echo's of variables in the writing function, when you send a message to someone, so it could output directly on the screen what kind of data is saved. I'm not sure where the hiss happens. Definitely, at the step when a new array is created, may be the script just takes/inludes the older cached version of the file... it could be truly about kind of cache.

Yes, 2 is correct. I am debugging at the moment. A caching problem crossed my mind too. I'll contact my webhost for that. As soon as I find anything I'll let you know. Thanks a lot for looking into this with me!

You're welcome. Report here whatever is solved or discovered - and if that would be about caching, switching to mysql mode would be the easiest solution for you.

Update about my problem:

I found out that when I send a PM from the mobile forum then the $inboxtotal etc. become correct! So settings.php is written correctly then!
So we are getting somewhere now. But since I handed over the problem to my webhost yesterday, they will start investigating this on Monday (and I really appreciate that they are willing to deep into this), I am in doubt about what to do now: shall I cancel the ticket or shall I send them the above clue and let them investigate? I'm not sure if it's my problem or an environmental problem. What would you do?

From the technical point of view, there is no difference either you use the mobile mode or the desktop mode, because for both modes, there is absolutely the same script with the same functions. However if you use the older mobile mode for miniBB (as I suppose), it has been installed on a subdomain, right? If so, here could be some key to the problem, as sort of caching could be enabled for your primary domain and disabled for a sub-domain.

For me, kind of caching of files still seems more obvious reason, so I'd wait for the hoster's investigation.

Ok thanks. And yes, it is a subdomain.

The webhost found the solution!
The mobile subdomain runs under PHP 5.4 but the main domain runs under PHP 5.6. I never realized that they run under different PHP-versions. Apparently our PM service doesn't run smoothly under 5.6. The webhost changed the PHP-version on the main domain back to 5.4 and now the PM service runs correctly again. I think I should upgrade to your latest PM service addon.

That's curious to know, thanks... I suppose, the newest mobile version will not require a standalone sub-domain anymore, so the problems will be the same for both versions in the future :) However I'm still wondering how PHP could be involved in that... file caching or something... The functions we use for this flat-files PM module, are absolutely native and very transparent, nothing about specific classes and libraries... that's actually would be the proper solution to investigate: why PHP 5.6 handles the files differently other than 5.4.

But if you're happy so far with an existing solution, that's fine for me.

You could get the latest version from your Customers area, and I suppose, this is the right time to get it for your older miniBB version, since the newer PM module for miniBB 3.3 released this summer, will have even more incompatibilities regarding the older miniBB versions. However, like I wrote above, most functions in the current PM module were not about messages saving principles, but mostly about compatibility issues with the newest miniBB versions, and there were also no security patches. I suppose, because of that, you could still stay with your older add-on so far.

Many hosts allow to change PHP Version.

I'd do this -

1: Change PHP version back to PHP5.6 (or ask your host to do it),
2: DISABLE OPCACHE (most hosts have it enabled in PHP5.5+).

OpCache is most likely the problem. Report back :

Ok sounds interesting. I just sent my webhost an email to ask to turn off opcache. I cannot turn it off myself, it's read-only (and indeed opcache.enable=on). I'll report back to you. Thanks.

Adding this to .htaccess may disable opcache (depending on server though):

Yesss, problem solved!

They turned off opcache under PHP 5.6 and now the PM module (2.4.5) works again. Strange thing though is that under PHP 5.5 the PM module worked fine, even with opcache turned on. So there seems to be a difference between the opcache implementation in 5.5 and 5.6 from my point of view. Either way, I'm happy this problem is solved thanks to you guys.

So it still was about the caching issue like expected initially... Glad to know it's solved, and thanks to tom322, at first...

tom322 - how did you come to the solution, did you experience something similar with your scripts in the past?

Yes, the same problem may appear with the Checker addon (that's how I discovered it). Good news is that it is possible to exclude Opcache from specific folders (but a regular shared host may not be willing to do it just for your site), so even PM-flat-files / Checker addons can work with Opcache enabled.

Opcache may be useful for static pages but in general a dynamic forum doesn't need this enabled. I don't have it enabled on a relatively large forum and see no difference in speed if I enabled it.