OWA Client page error will be similar to this:
Webpage error details
User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; InfoPath.2; Tablet PC 2.0; MS-RTC LM 8 )
Timestamp: Wed, 7 Oct 2009 15:36:45 UTC
Message: Syntax error
Line: 6
Char: 1
Code: 0
URI: https://mail.domain.com/owa/8.1.393.1/scripts/premium/flogon.js
The C:\Program Files\Microsoft\Exchange Server\ClientAccess\OWA\8.1.393.1\scripts basic and premium folders were missing files. I copied the files from the 8.1.375.2 folder into the 8.1.393.1\ basic and premium folder, but did not overwrite any files with a newer date. Verified that subfolders in folder 8.1.393.1 were not missing additional files.
I then went into IIS – Default Web Site – OWA – 8.1.393.1 – Directory Security Tab – Authentication and enabled anonymous access for IUSR_SCO-Exchange.