I tried to check and solve the problem.

First, I checked robots.txt - the access to media folder is allowed

Then I checked .htaccess . Therfore I compared two physically separated installations (both Version 6.10.6) for .htaccess and found out, that the installations differ.

Installation A
+ /blogs/media/.htaccess

- /html/blogs/rsc/.htaccess
- /html/blogs/locales/.htaccess
- /html/blogs/skins/.htaccess
- /html/blogs/conf/.htaccess
- /html/blogs/inc/.htaccess
-/html/blogs/skins_email/.htaccess

Installation B
- /blogs/media/.htaccess

I assume, that htaccess in media is not correct.
But i don't know how it behaves with the other files missing in installation A

Could you deliver a list of folders that should have a .htaccess?

Thanks and regards, Will

First check the summary view ( https://b2evolution.net/man/summary-view ) and check what this shows. If the image is not there, then it's not properly configured and b2evo will not try to pass it to twitter.

@fplanque I deleted the .htaccess file in /media folder and now twitter cards work fine.

But please tell me, what I should do towards the different use of .htaccess file in two installations, each with version 6.10.6

Thanks and Regards, Will

@saunders it is recommended you properly set up all .htaccess files in all relevant folders: https://b2evolution.net/man/htaccess-files

@fplanque the information provided at that page your link referrs to, mentions to set .htaccess e.g. to /media. But that would - as I now experieced - cause, that twitter cards don't really work, because Twitter could not show images from the page, to which a tweet links.
To ensure that not every service etc. have access to /media folder, after I deleted the .htaccess file in this folder, I edited robot.txt:and adapted it this way:

User-agent: *
Disallow: /blogs/media/

User-agent:Twitterbot
Allow: /blogs/media/blogs/

That is not correct. You have to use the correct media/sample.htaccess from the b2evolution distribution.

See Warning here: https://b2evolution.net/man/htaccess-files

For the media folder, the correct file contents are:

# We don't want any PHP execution in this folder! (Prevent direct invocation of PHP files)
<IfModule mod_php5.c>
    php_flag engine off
</IfModule>

<IfModule mod_rewrite.c>
    RewriteEngine On

    # Redirect any _evocache file that does not exist to generate new by htsrv/getfile.php:
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{QUERY_STRING} mtime=(\d+)
    RewriteRule ^(.+/[_\.]evocache/.+/([a-z0-9\-]+)\.[a-z]{3,4})$ /htsrv/getfile.php?abspath=$1&size=$2&mtime=%1 [L]
</IfModule>

@fplanque I did not add any PHP execution in a file or folder on b2evolution. I just added lines to robot.txt in the root directory
But I loaded the .htaccess file as recommende to /media

I have other lines in robot.txt:

User-agent: *
Disallow: /blogs/media/
Disallow: /blogs/htsrv/
Disallow: /blogs/xmlsrv/
Disallow: /blogs/?disp=msgform

should I delete these?

OK, sorry I did make a confusion by reading quickly but make sure you understand this:

• robot.txt is for kindly asking third parties to not index files you don't want them to index. This is a "nice to have" setting.
• .htaccess is for protecting your folders against potential hacks. This is a "Highly recommended for best security" setting.

@fplanque yes, I know; to disallow /media by robot.txt should prevent my images to be crawled by google. google - up to now - respects robot.txt, also bing and e.g. twitter does - other search engines and most of the social services don't bother me.
Thanks for your support.