Replace
return $userAgent.' ('.implode('; ', $agentInfo).')';
with something like
return "Mozilla/5.0 (X11; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0";
(the user agent of a browser, that displays the feed correctly)

I replaced the line, but the error remains. Then I changed the FF version from 54.0 to 51.0 but it is still not working.
Any other ideas?

I am using selfoss 2.16 and the standard "RSS feed" spout.

In which way should I change the helpers/WebClient.php?

Replace

return $userAgent.' ('.implode('; ', $agentInfo).')';

with something like

return "Mozilla/5.0 (X11; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0";

(the user agent of a browser, that displays the feed correctly)

What version of selfoss do you use and which spout? 403 is server error, joomgallery could be blocking you. If you use “RSS feed” spout you could try changing helpers/WebClient.php to use different user agent.

I am using selfoss 2.16 and the standard "RSS feed" spout.

In which way should I change the helpers/WebClient.php?

I won’t be able to help you with this. You will have to contact the fora admins.

No problem.
I will try to do this. But I have only little hope after reading a lot of threads at xda-developers.

I still get an error message.
The complete error message is:
error loading feed content: cURL error 22: The requested URL returned error: 403 Forbidden

Any ideas?

What version of selfoss do you use and which spout? 403 is server error, joomgallery could be blocking you. If you use “RSS feed” spout you could try changing helpers/WebClient.php to use different user agent.

In the xda-developers forums there are a lot of threads about the rss topic, but none has a solution.

I won’t be able to help you with this. You will have to contact the fora admins.

If I try to include http://www.forum.joomgallery.net/index.php?type=rss;action=.xml as source, I am able to save it as rss feed but when I want to update the feed I always get an Error 403 Forbidden.

This source works for me just fine.

I still get an error message.
The complete error message is:
error loading feed content: cURL error 22: The requested URL returned error: 403 Forbidden

Any ideas?

I found the following feed:
https://forum.xda-developers.com/external.php?type=RSS2

I found the same one, but got the same error messages.

In the xda-developers forums there are a lot of threads about the rss topic, but none has a solution.

If I try to include http://www.forum.joomgallery.net/index.php?type=rss;action=.xml as source, I am able to save it as rss feed but when I want to update the feed I always get an Error 403 Forbidden.

This source works for me just fine.

Another problem is the rss feed from xda-developers.com.
I am not able to include the feed in selfoss. Is there a way to add a forum or thread from xda-developers as rss feed?

I found the following feed:

https://forum.xda-developers.com/external.php?type=RSS2

but it prints

Must be logged in to get external feeds.

and after I log in, it just shows

Forbidden

Not much we can do about it, you should write the forum support.

I am running selfoss on a shared web hosting server in a subdomain. Many feeds are already included but with two sites I am still struggling.
If I try to include http://www.forum.joomgallery.net/index.php?type=rss;action=.xml as source, I am able to save it as rss feed but when I want to update the feed I always get an Error 403 Forbidden.
Another problem is the rss feed from xda-developers.com.
I am not able to include the feed in selfoss. Is there a way to add a forum or thread from xda-developers as rss feed?

I tried several ways of rss feeds, but I still get the same error messages.

Kind regards
Martin