No response on http during compacting databases

During compacting of databases, the http taks does not responding anymore.Also on the dominoconsole appears a strange message :

WAITING FOR READ LOCK ON FRWSEM 0x0244 database semaphore (@ADAB970C) (/local/lotus/data/apps/database.nsf) (R=0,W=1,WRITER=15090:-1237543232,1STREADER=00000:00000) FOR 30000 ms

any idea ?