A 406 not acceptable error is generally related to Suhosin "keeping you safe" (your host has a rule that is being triggered), please look in your error logs for the 406 error and relay the lines here.

You should probably also run the System Requirements Test and it'll relay any settings.

I am a Web Development Contractor, I do not work for UBBCentral. I have provided free User to User Support since the beginning of these support forums.
Do you need Forum Install or Upgrade Services?
Forums: A Gardeners Forum
UBB.threads: UBBWiki, UBB Styles, UBB.Sitemaps
Longtime Supporter & Resident Post-A-Holic
VNC Web Services: Code Modifications, Upgrades, Styling, Coding Services, Disaster Recovery, and more!