You should verify if the dll's in your bin folder are the ones from the Sitecore version you are on (8.0). Errors like this happen very often and are almost always caused by dll's in the bin folder that are still from the old Sitecore version.
Download the zip of a clean Sitecore with the exact version you are on. Unzip and compare the dll's in the bin with your bin. You can do the compare manually (can, but won't) or use any compare tool you are used to.
If you find any inconsistencies, overwrite the dll in your bin folder with the one from the clean Sitecore.
Also, make sure that if you deploy to your environment no dll's are overwritten again. Check your references (or update if using Nuget - which you should) and sometimes it is also needed to clean (delete) all bin and obj folders hanging around in your projects.