I have to admit that this is beyond my direct expertise, but have you read this?
[quote]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
Squid only knows about standardized HTTP request methods.
You can add up to 20 additional “extension” methods here.
extension_methods REPORT MERGE MKACTIVITY CHECKOUT
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