F5 LTM SharePoint 403 Error

Hi,

I'm using 2 F5 BigIP LTM (Version 11.2.1) to perform a load-balancing for a MS SharePoint 2010 Farm. I randomly get a HTTP 403 Error at some Client-PCs. This error occurs at random times. At some time it happend every day and then there is nothing over weeks. Only single users have this problem. Most of the time the Problem is solved by closing the Browser (IE 9) and waiting some time. Are there some logfiles to investigate an how can I find them or does someone know a propper solution for this problem?

Thank you for your help!

guettmi