EDIT - NVM, ALREADY REPORTED:
This is a very persistent bug.
It started as soon as I tried to edit this horse model’s animations:
dispeller’s Epic Horse.rbxm (11.8 KB)
I was having some trouble with one of my projects, so I reached out for help.
Why are my custom animations slow?
If anyone edits the horse in that file I’ve uploaded, their Animator plugin breaks.
I’ve had a friend tell me it also broke for him when he tried.
I don’t remember completely, but I don’t think this error occurred before I uploaded the file.
Previous to uploading the file, I was able to edit the horse’s animations without a problem.
After you get the error once, it seems to keep occurring no matter what. Even with other models, not just the horse.
The worst part is the persistence of this bug.
I’ve tried
- Uninstalling and reinstalling Roblox Studio
- Disabling and reenabling the Animation Editor plugin
- Uninstalling the Animation Editor plugin and reloading studio to get it back
The only thing I haven’t tried is clicking the big jucy Install button on the Animator Plugin’s library page.
There’s a warning saying I should use the version of the Animation Editor that comes with Roblox Studio, but it’s really tempting to ignore that warning. 
Just to further clarify, the GUI shows up. I get the error whenever I press the “Create” button.
Console Errors:
> 01:43:12.883 - Invalid animation id '<error: unknown AssetId protocol>':
> 01:43:12.885 - Stack Begin
> 01:43:12.885 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.DataModels.DataModelPlayState', Line 100 - function loadAnimation
> 01:43:12.886 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.DataModels.DataModelPlayState', Line 134 - function recreateAnimationTrack
> 01:43:12.887 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.DataModels.DataModelPlayState', Line 165
> 01:43:12.887 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.UtilityScripts.UtilityScriptEvent', Line 61 - function fire
> 01:43:12.888 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.DataModels.DataModelSession', Line 591 - function endResetAnimation
> 01:43:12.888 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.DataModels.DataModelSession', Line 626 - function resetAnimation
> 01:43:12.889 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.UtilityScripts.UtilityScriptPluginSetup', Line 328 - function populateEditor
> 01:43:12.890 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.UtilityScripts.UtilityScriptRigSelection', Line 186 - function showEditor
> 01:43:12.890 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.UtilityScripts.UtilityScriptRigSelection', Line 216
> 01:43:12.891 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.GUIScripts.GUIScriptDarkCover', Line 10
> 01:43:12.892 - Stack End
> 01:43:14.298 - Invalid animation id '<error: unknown AssetId protocol>':
> 01:43:14.298 - Stack Begin
> 01:43:14.299 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.DataModels.DataModelPlayState', Line 100 - function loadAnimation
> 01:43:14.299 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.DataModels.DataModelPlayState', Line 134 - function recreateAnimationTrack
> 01:43:14.300 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.DataModels.DataModelPlayState', Line 165
> 01:43:14.300 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.UtilityScripts.UtilityScriptEvent', Line 61 - function fire
> 01:43:14.301 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.DataModels.DataModelSession', Line 591 - function endResetAnimation
> 01:43:14.301 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.DataModels.DataModelSession', Line 626 - function resetAnimation
> 01:43:14.302 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.UtilityScripts.UtilityScriptPluginSetup', Line 260 - function exitPlugin
> 01:43:14.303 - Script 'Plugin_AnimationEditor.rbxm.AnimationEditorRigBuilder.AnimationEditor.Code.UtilityScripts.UtilityScriptPluginSetup', Line 60
> 01:43:14.303 - Stack End
> 01:43:14.304 - Invalid animation id '<error: unknown AssetId protocol>':
Related posts I found:
Animation editor wont let me animate any rigs
Animations no longer work with rbxassetid:// Ids
These posts seem to be having a very similar problem, but not quite the same error.
Feel free to ask me for further clarification.