CB had some problems with MOL becoming unresponsive after uploading files using the eXist admin client. I restarted MOL using the tomcat manager (took forever, but it worked - without restarting tomcat itself!) and things seem to be back to normal, but if CB uploads another file we end up in the same unresponsive state. While in that state, tailing the exist.log shows numerous entries like this:
DEBUG (BrokerPool.java [get]:1408) - waiting for a broker to become available
A quick google finds this page http://sourceforge.net/apps/trac/exist/browser/trunk/eXist/src/org/exist/storage/BrokerPool.java?rev=15806 - not sure if this is current enough to be useful...

Thoughts for resolution:
CB has had permission to upload for several days, and has had no problem until today.
CB changed his password in exist today. The two may be connected.
Is the problem is that CB changed the password himself? Would the problem occur if the admin user makes password changes for non-admin users?

NOTE: tailing /usr/local/webapps-dev/mapoflondon/WEB-INF/logs/exist.log shows that every request/response has a debug entry. Is logging set too sensitively for production?