What could provoke those problems please ?

[Anceane]

Any idea what it means please ?

2015-08-18T18:08:52.218-05:00 |cff0000Lua Error: assertion failed
stack traceback:
	[C]: in function 'assert'
	EsoUI/Libraries/ZO_KeybindStrip/ZO_KeybindStrip.lua:174: in function 'ZO_KeybindStrip:AddKeybindButton'
	EsoUI/Libraries/ZO_KeybindStrip/ZO_KeybindStrip.lua:287: in function 'ZO_KeybindStrip:AddKeybindButtonGroup'
	EsoUI/Ingame/Mail/MailInbox.lua:33: in function 'callback'
	EsoUI/Libraries/Utility/ZO_CallbackObject.lua:104: in function 'ZO_CallbackObject:FireCallbacks'
	EsoUI/Libraries/ZO_Scene/ZO_Scene.lua:248: in function 'ZO_Scene:SetState'
	EsoUI/Libraries/ZO_Scene/ZO_SceneManager.lua:155: in function 'ZO_SceneManager:ShowScene'
	EsoUI/Libraries/ZO_Scene/ZO_SceneManager.lua:430: in function 'ZO_SceneManager:OnSceneStateChange'
	EsoUI/Ingame/Scenes/IngameSceneManager.lua:278: in function 'ZO_IngameSceneManager:OnSceneStateChange'
	EsoUI/Libraries/ZO_Scene/ZO_Scene.lua:249: in function 'ZO_Scene:SetState'|r

And while i am asking, do you have any idea of what could cause this : this happens mainly when i use the mail, or the bank, or want to check the achievements. The screen goes like this and the game freeze and the connection with the game is slowed until it stops completely for the majority of the time, but it happens also that suddenly the game recover and the screen is back to normal.
This affect only the game, my computer does not freeze and i can go on the internet without any probleme.