Hello Andi,
I asked my hosting provider to look into the problem, as I thought there was a problem with the hosting settings
His answer:
No, the problem is other servers don't have such a security as ours. Your Wiki script seems inompatible with mod_security
standards, here's the error in your error.log:
[Sat Nov 14 11:07:41 2009] [error] [client 62.235.139.238] ModSecurity:
[file "/etc/httpd/modsecurity.d/10_asl_rules.conf"] [line "718"] [id
"340162"] [rev "170"] [msg "Atomicorp.com WAF Rules: Remote File Injection
attempt in ARGS (AE)"] [data ""] [severity "CRITICAL"] Access denied with
code 403 (phase 2). Match of "beginsWith http://%{SERVER_NAME}/" against
"MATCHED_VAR" required. [hostname "test.hostamill.be"] [uri
"/lib/exe/fetch.php"] [unique_id "8mPmR1UZjNkAAHmQZmMAAAAN"]
More info:
http://www.dokuwiki.org/faq:mod_security