In Exchange 2010 OWA can run into odd problems if the user has a large number of Outlook rules which exceed the default allocated space, which is 64KB. Specifically, an error like this will show up in the logs and the user won't be able to turn Out of Office (OOO) on or off when using OWA - it works fine with desktop Outlook.
The Rules quota of mailbox 'user.name'@msi.co.uk has been reached and the automatic reply rules can't be enabled or updated. Delete some existing rules or increase the user's rule quota and try to set the automatic reply again. You can use the Set-Mailbox cmdlet to increase a user's rules quota.
Note that this issue is not, explicitly, the number of rules a user has set but the space they take up - so perhaps a measure of their complexity. The first command sets the users space limit from the default 64KB to 256KB, and the second command confirms it has worked.
[PS] C:Windowssystem32>Set-Mailbox -Identity 'username' -RulesQuota "256 KB"
[PS] C:Windowssystem32>Get-Mailbox -ResultSize unlimited | Format-Table Name,RulesQuota -
Name RulesQuota
---- ----------
***************** 64 KB (65,536 bytes)
'user.nanme' 256 KB (262,144 bytes)
DiscoverySearchMailbox {D919BA05-46A6-415f-80AD-7E09334BB852} 64 KB (65,536 bytes)