thanks for the response.
I investigated and found that feed provider adds 500 error as response header, but the content is valid.
I print the headers returned in the “function fetch_file_contents” of the “include/functions.php” file:
HTTP/1.1 500 Internal Server Error
Expires: Tue, 02 Jan 2018 16:44:48 GMT
Set-Cookie: ASPSESSIONIDQCRTBQDA=KCKMNOKBCCBAPDOLLAMGJLIK; path=/
Date: Tue, 02 Jan 2018 16:29:47 GMT
Content-Length and Content-Type are valid, and payload is a regular XML.
I believe this is a very rare case, I don’t expect that tt-rss manages it. In the meanwhile I will arrange a workaround on my instance.