Braindump

Next, you need to make sure the proxy server itself supports all the HTTP methods Subversion uses. Some proxy servers do not support these methods by default: PROPFIND, REPORT, MERGE, MKACTIVITY, CHECKOUT. In general, solving this depends on the particular proxy software. For Squid, the config option is

# TAG: extension_methods
# Squid only knows about standardized HTTP request methods.
# You can add up to 20 additional "extension" methods here.
#
#Default:
# none
extension_methods REPORT MERGE MKACTIVITY CHECKOUT

(Squid 2.4 and later already knows about PROPFIND.)