On my website: http://www.bondsscheidsrechter.nl there is the following text:

"This page cannot be displayed due to an internal error.

If you are the administrator of this site, please visit the XOOPS Troubleshooting Page for assistance.

Error [Xoops]: Unable to connect to database in file class/database/databasefactory.php line 34"

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So what I did was look up databasefactory.php:
Line 33/34:

if (!$instance->connect()) {
trigger_error("Unable to connect to database", E_USER_ERROR);

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I think I use the newest XOOPS version. What are my options? Thank you very much in advance!

The error probably means exactly what it says.

Can you access the same database using phpMyAdmin?

as Dave says, check if u can connect to the database using another program..

also, check your mainfile.php

see if the MySQL settings are correct..

ie.. host name,user,password