Why does Svn re-request authorization?

Question

subversion

There is libapache2-svn_1.8.5-1+WANdisco_amd64 running on Ubuntu 13.10 running apache dav_svn. Authorization is configured through the AuthExternal pwauth module to log in under system accounts. The problem is the following: upon checkout or update by the tortoisesvn client (did not try others), after downloading a certain number of files, the server re-requests the previously saved authorization, which it does not accept. Only after canceling the session, cleaning through Clean up and restarting Update, the history repeats itself. Can anyone come across this?