As this are achievments try to disable Achievement related addons like the Pithka Achievememt tracker.
Or check it settings!
btw: Once DebugLogViewer is enabled it should show all chat mesages of addons too?
Not sure why it doesn't for you.
Check the settings of DebugLogViewer -> LibDebugLogger -> Enable stack tarceback and switch the tracked level to e.g. Debug.
Clear the log via the buttons there.
Reloadui.
Then have the messages show up again, they should not be in the chat anymore but in DebugLogViewer now.
Click one of the messages and see (from bottom to top) where it comes from.
e.g. copy the whole text of that opup showing the stack trace here, so we can see and help find the addon name, if you cannot see it.
Or even better:
If DebugLogViewer shows the message, do the following:
-Note date and time it happens!
-ReloadUI to save all proper to live/SavedVariables/LibDebugLogger.lua
-Upload live/SavedVariables/LibDebugLogger.lua here:
https://sir.insidi.at/or/logviewer/ (drag & drop, then click top right share and send us the link created here!)
-Tell us date and time the mesage happened so we can have a look at the debuglog file ourselves
More information about lua error message reading (stack traceback), LibDebugLogger file sharing, can be found here:
https://www.esoui.com/forums/showthread.php?t=8858
-> lua error messages ingame - How do I read them?
-> How to report a lua error message/a bug to the developers