Hi all,
A while ago, Sourceforge introduced new URLs for their Subversion repositories.
Support for the old-style URLs (without the project name prefix) has now been dropped.
The code for OpenMPT is now here:
https://modplug.svn.sourceforge.net/svnroot/modplug/trunk/OpenMPT
If you already have a working copy of the code pointing to the old repository location, you can make it point to the new URL with TortoiseSVN.
Simply right click your project top level directory in Windows Explorer, and in the TortoiseSVN sub-menu, select Relocate....