Well, your error is:
Data too long for column 'ONLINE_AGENT'
So the UA string is too long to be put into the db field:
Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0; GTB5; SU 3.22; SU 3.14; SLCC1; .NET CLR 2.0.50727; Media Center PC 5.0; Zune 3.0; .NET CLR 3.5.30729; OfficeLiveConnector.1.3; OfficeLivePatch.0.0; .NET CLR 3.0.30729; .NET CLR 1.1.4322; Creative AutoUpdate v1.10.10)
Basically, the user has a holy plethora of crap installed on their computer and it's all attaching to the UA string. The limit for this field is 255 characters, his UA is more than that (280).
I think we need to strip the input on that field to make sure it's never longer than 255 characters or change the field type to a text vs varchar.