I have a server with svn repository with http access with path-authentication.
That would get the message I need to try to accomplish do not allow the action (r, w, rw).
(i.e. the directory to which I have read, but do not have the right to write — specially checked and got a similar message: Server sent unexpected return value (403 Forbidden) in response to MKACTIVITY request for '/repos/project/!svn/act/7563e694-d243-11df-8ea3-cbb8be9477e5')
Maybe you have just not enough right.
As an option, maybe not the rights to access the directories trunk, but at the same time, the operation may be permitted in the subdirectory. Ie, it makes sense to try to specify a subdirectory of trunk.