The Apache 501 "Not Implemented" error is caused by a regression bug in the subversion 1.2.1 libraries. The "svn st -u" command of subversion 1.2.1 is incompatible with subversion 1.1.x servers. This normally happens when you "check for modifications" against old servers in TortoiseSVN.
TortoiseSVN can't do anything about this.
You can either:
- upgrade the server to subversion 1.2.1
- downgrade the client to 1.2.0 or earlier
- don't use the "check for modifications" dialog in TortoiseSVN against old servers.
- wait until the subversion team releases the 1.2.2 libraries.