Where can I find the Apache error log?

If your server is Windows-based, typically in C:\\Program Files\\Apache Group\\Apache2\\logs.

If it's Un*x, try /var/log/httpd.

You will need to check the Apache error log yourself when you receive errors from eg. the Repo-Browser that looks like this: 'PROPFIND: Request failed - HTTP xxx'. The Apache error log contains a message describing the cause for these errors. The real error messages are unfortunately not propagated to Subversion clients like TortoiseSVN.