See https://github.com/Gethe/wow-ui-source/blob/ptr/Interface/AddOns/Blizzard_Deprecated/Deprecated_9_1_5.lua with a list of deprecated methods. According to BugSack, the addon now throws after GetBackdrop() call in modules/misc/tooltips.lua in AdjustBackdrop function (there may be other places as well).
Simple way to repro (for me) was to do a mouse-over on my character portrait - from my brief venture into the code, it probably triggers a recolor of the portrait based on faction allegiance.

I am not currently subscribed to World of Warcraft and have no intentions of doing so any time soon.

Accepting PR's for this.

Edit: I should probably update Curse and WoWInterface with this info, I didn't even realise 9.1.5 was out now.

I had a similar error for TBC Classic, I solved it by replacing the included AceGUI-3.0-SharedMediaWidgets in ImprovedBlizzardUI\libs

With the version found here WoWAce

Haven't tested if it works in Retail, but it solved it for TBCC at least, without losing any functionality from ImpBlizzUI.

I had a similar error for TBC Classic, I solved it by replacing the included AceGUI-3.0-SharedMediaWidgets in ImprovedBlizzardUI\libs

With the version found here WoWAce

Haven't tested if it works in Retail, but it solved it for TBCC at least, without losing any functionality from ImpBlizzUI.

Does not solve the issue here, at least it did not for me.