Start fop2_server in debug mode, level 511:
service fop2 stop
cd /usr/local/fop2
script capture.log
./fop2_server -X 511
(try to delete a voicemail)
ctrl-C
exit
Look at the log file for clues. It could be some restriction in the mailbox/context name (coded in fop2), or perhaps permission issues or nfs bugs/misconfigurations that will be kind of hard to debug.