When running Sanctum of Domination Mythic I received this error:

DBM is fully up-to-date as of the time of reporting this bug.

1x [ADDON_ACTION_FORBIDDEN] AddOn 'DBM-Core' tried to call the protected function 'CancelScene()'.
[string "@!BugGrabber/BugGrabber.lua"]:481: in function <!BugGrabber/BugGrabber.lua:481>
[string "=[C]"]: in function `CancelScene'
[string "@FrameXML/CinematicFrame.lua"]:193: in function `CinematicFrame_CancelCinematic'
[string "@DBM-Core/DBM-Core.lua"]:6910: in function `handler'
[string "@DBM-Core/DBM-Core.lua"]:1055: in function <DBM-Core/DBM-Core.lua:1042>

Locals:
_ = Frame {
 RegisterEvent = <function> defined @!BugGrabber/BugGrabber.lua:487
 0 = <userdata>
 UnregisterEvent = <function> defined @!BugGrabber/BugGrabber.lua:487
 SetScript = <function> defined @!BugGrabber/BugGrabber.lua:487
}
event = "ADDON_ACTION_FORBIDDEN"
events = <table> {
 ADDON_ACTION_BLOCKED = <function> defined @!BugGrabber/BugGrabber.lua:553
 ADDON_ACTION_FORBIDDEN = <function> defined @!BugGrabber/BugGrabber.lua:553
 PLAYER_LOGIN = <function> defined @!BugGrabber/BugGrabber.lua:547
 LUA_WARNING = <function> defined @!BugGrabber/BugGrabber.lua:562
 ADDON_LOADED = <function> defined @!BugGrabber/BugGrabber.lua:507
}

Even though this error is being thrown, the cinematic skips were still successful for me. Is simply the error's presence enough for you to want to remove the functionality?

The error actually encourages users to disable DBM. An uninformed user will consider this a problem with DBM and continue opening issues with us, or worse, uninstall mod thinking it's causing problems with their game. So until blizzard fixes how much they broke with Stanzilla/WoWUIBugs#483 , the feature is just retired.

Yeah, blizzard decided randomly this week to make it a protected function. I just killed off DBMs feature to auto cancel but scenes during fights. users are gonna have to manually skip them from now on.

It's still skip scenes using PLAY_MOVIE api but that's mostly older stuff that uses CGI instead (like lich king scene)

Newer stuff that uses real time rendering and CINEMATIC_START and that's what they added protection to.

The same showed up on the Sylvanas fight.

Was CancelScene() actually protected or is it a random Blizzard victim?

Was CancelScene() actually protected or is it a random Blizzard victim?

It was not protected, just a bug as mentioned in the message before yours. Some addons are disabling/removing their Cinematic/Cutscene skip functionality temporarily, to prevent errors and additional user reports.