Ahh, well, I can definitely say, that your error doesn't comew from any of the mentioned mods, but one of the new EE only mods that cheese your thief01.2da file, just a small look at it from Notepad++ can tell you that, it's full off cheese:2DA V1.0 **** 1 2 ... 40 ABILITY1 GA_SPCL412 .... ... **** ABILITY2 **** .... ... **** ABILITY3 **** .... ... **** ABILITY4 **** .... ... **** ABILITY5 **** .... ... **** ABILITY6 **** .... ... **** ABILITY **** **** ... AP_D5_DUAFC AP_D5_DUAFC AP_D5_DUAFC AP_D5_DUAFC AP_D5_DUAFC AP_D5_DUAFC AP_D5_DUAFC AP_D5_DUAFC AP_D5_DUAFC AP_D5_DUAFC AP_D5_DUAFC AP_D5_DUAFCSo, it's like it's intentionally made to reallly F's your MMBM. And quick search reveals the mod being in question, is Tweaks Anthology.
Thank you for pointing me in the right direction. Digging around with NearInfinity strongly suggests that this "ability" is pointing at a null, because I can't find it defined anywhere, even though it's referenced, a whole bunch of times, in the druid, mage, priest, and thief clabxx01.2da files.
And one test later, the offending item in Tweaks Anthology looks to be the "Enforce PnP Proficiency Rules on Dual-Classed CharactersEnforce PnP Proficiency Rules on Dual-Classed Characters" component. I can't tell how it works, so I'm not sure if the execution here could be improved or if they butted up against an engine limitation, but either way, this is getting axed for now.
Thank you again, Herra Mikkola!