I'm getting this error for al my users does anybody know wat the problem is and how i can fix it

pop3(bas@giskit.nl): Mar 06 09:10:37 Error: file_wait_lock() failed with index file /mailstore/vmail/giskit.nl/bas/.INBOX/.imap.index: No locks available

Met vriendelijk groet,

Bas van Dikkenberg

GISkit bv

BFVD1-RIPE

Tel: +3130-6340430

Fax: +3130-6342433

Prive Tel: +3130-6372769

Mob: +316-52384841