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If you haven't already done so, close your site to the public and upgrade your site to v2.0.7.1. This version fixes know vulnerabilities in Xoops.
After you've successfully upgraded your site, log into your FTP account and chmod all folders on XOOPS to 555.
Exceptoins:
chmod the following as 777
/cache
/templates_c
/uploads
chmod the following 444
mainfile.php
If you're not familiar with chmod, here's what the values mean:
777 = read/write/excecute by all
555 = read/excecute by all
444 = read by all
You should also check the phpBB site / documentation to ensure that your forum is the most recent version, and the permissions on your forum's files are correct. The controls in the Admin panel are not the only factors you need to consider.
This kinda thing is more common on shared hosting plans, so as mentioned above, contact your host.