Backuppc Error: Unable to connect to BackupPC server

[010101]

I just logged into my backuppc via the web UI and I got this error

Error: Unable to connect to BackupPC server

backuppc wasn't running

This is the log, see the last line

2011-04-26 01:00:00 Running 2 BackupPC_nightly jobs from 0..15 (out of 0..15)
2011-04-26 01:00:00 Running BackupPC_nightly -m 0 127 (pid=653)
2011-04-26 01:00:00 Running BackupPC_nightly 128 255 (pid=654)
2011-04-26 01:00:00 Next wakeup is 2011-04-26 02:00:00
2011-04-26 01:00:02 Started full backup on s1.domain.com (pid=655, share=/)
2011-04-26 01:11:50 BackupPC_nightly now running BackupPC_sendEmail
2011-04-26 01:11:51 Finished  admin1  (BackupPC_nightly 128 255)
2011-04-26 01:11:52 Finished  admin  (BackupPC_nightly -m 0 127)
2011-04-26 01:11:52 Pool nightly clean removed 0 files of size 0.00GB
2011-04-26 01:11:52 Pool is 0.00GB, 0 files (0 repeated, 0 max chain, 0 max links), 1 directories
2011-04-26 01:11:52 Cpool nightly clean removed 8352 files of size 1.77GB
2011-04-26 01:11:52 Cpool is 21.04GB, 453656 files (1 repeated, 1 max chain, 15401 max links), 4369 directories
2011-04-26 02:00:08 Next wakeup is 2011-04-26 03:00:01
2011-04-26 03:00:02 Next wakeup is 2011-04-26 04:00:00
2011-04-26 04:00:00 Next wakeup is 2011-04-26 05:00:00
Can't fork at /usr/share/backuppc/lib/BackupPC/Lib.pm line 1313.

this line reads:

if ( !defined($pid = open(CHILD, "-|")) ) {

At 01:00 a full backup started and it lasted 257.4 minutes ( more than 4 hours, which compared to the previous full one - 237min - sounds regular)

I manually restarted it and now it's running fine, but I'd like to understand what happened.

Anyone has a clue?