The nop-template plugin AjaxFilters was out of date.
I downloaded it right after 3.8 hit, but after the 3.8 release nopcommerce did a bugfix for this method.
I downloaded the latest plugin again and replaced everything.
Now its working fine.
Unfortunately, that didn't work for me.
I had the bug error appearing for one of my plugins, then I learned about the bugfix to the 3.80 version and replaced my /bin and Administration views. After that, all pages of my site had the error... So I downloaded the nop-templates plugins and replaced them on my site...same error persists...
I believe the problem is specific to the 7spikes plugins or theme because my other store on the same installation which uses DefaultClean theme and no plugins is loading with no errors.
I guess I should ask... What is the correct procedure for applying the bugfix? Which files must be replaced? Does the upgrade script need to be re-executed?
I screwed up and did this on a production site, so I really need to get it back up and running ASAP.