gallery not really working :(

The point is: I should delete all files completely and reinstall them freshly piece by piece. That's the only way I know to make sure that no bad or corrupted file may remain on my account.

I think you did not get me :) That is does not matter. It may be there no problem. Just make sure nothing was changed in working files. And then protect your site (htaccess) that only index.php can be accessible and on other files. In this case it does not matter if there is backdoors or not. No one will have access to them anyway.

Hm, that is new way to me, that nobody told me yet. You really think that I don't have to care about possible corrupted files as long as I make index.php only accessible?? Tell me more. I just do nothing understand ._.

You do no have to care about any extra file. I mean file that is not part of the system. And even any file of the Joomla and other extensions too in terms of backdoor. they will not work that way. But if someone modified your authorisation plugin and collect all user emails and password by sending copy to his email, the you have to prevent this. it is easy. Override joomla over you joomla root and all files will be replaced. update cobalt and rest.

I am 99% sure this will secure your files enough. Then htaccess protection will make the rest.

here is the example what you can place in htaccess

<Filesmatch "\.(php|zip|ini)$">

order deny,allow

deny from all

</Filesmatch>

<Filesmatch "^index.php">

order deny,allow

allow from all

</Filesmatch>

# hack for RT templates compression

<Files ~ "^css\-.*\.(php)$">

Allow from all

</Files>

<Files ~ "^js\-.*\.(php)$">

Allow from all

</Files>

I am still a little bit sceptic because of 2 points:

1. All advices that I got until now (including joomla.org and my hosting company) told me to completely delete all files in joomla directory.

2. I do not know enough about coding to understand your code that you provide nor am I able to check if your advice is accurate. Same as for base64 issue yesterday. I cannot find out who is right. I do not even understand the issue. All I can do is asking people and do some networking, just like mr postman who delivers a message from one expert to the other one ;)

By the way: I experience an odd issue on joomla global configuration: I cannot save anything there. It keeps wilily all old settings, regardless what I enter. After clicking in "Save" or "Save & Close" it does in both case only a refresh of the page. Neither saving the entered details nor closing the page

Coming back to that topic, I have an update: I just installed a complete new joomla installation an another account - everything worked fine - of course :) after that I imported database of mgt.nethotel.at. Now same issue as on mgt.nethotel.at! In my eyes, this is reveals, that this issue is not a matter of any corrupted files (because there are only clean, fresh original joomla files on that server) but of database! Furthermore thinking of the sql injection we experience last week on that database what's coming into your mind? Any helpful advices?

not a matter of any corrupted files

Nobody meant corrupted files as not working files. I meant that some files might had been modified do it behaves the same you do not see it but some extra code added does what you do not expect. For example I can modify authorizations plugin i Joomla so it continue work as usual but also send all passwords to my private email.

You had to replace files to exclude this particular behaviour.

not a matter of any corrupted files

Furthermore thinking of the sql injection we experience last week on that database what's coming into your mind? Any helpful advices?

Unfortunately I do not know what is wrong. You do not see the date or you have fatal errors or, .... I have no idea what problem you meet after transferring your site to new installation.