According to this it happens on the mobile website when trying to add a message on a user's talk page. The first time it happened seems to have been at 01:47 UTC on 26 February, and then dozens of times since. I haven't been able to reproduce the problem though.

This appears to have been caused by rEMFR7e23ce8db3d6: Decode an escaped data attribute before making an API request, passing undefined through decodeURIComponent() turns it into the string "undefined".

Change 273579 had a related patch set uploaded (by Florianschmidtwelzow):
Don't use decodeURIComponent on Undefined

https://gerrit.wikimedia.org/r/273579

Change 273579 merged by jenkins-bot:
Don't use decodeURIComponent on Undefined

https://gerrit.wikimedia.org/r/273579

Change 273711 had a related patch set uploaded (by Florianschmidtwelzow):
Don't use decodeURIComponent on Undefined

https://gerrit.wikimedia.org/r/273711

It would be great if the change above could be SWAT deployed (already cherry picked): Unfortunately, I'm not sure if I'm able to be online during the swat windows at Monday and Tuesday. Can I kindly ask, if someone else would check the change after deploying (the people who will be the "requesting developer")? :)

I've commented on the patch. I think we need to make a few follow ups before SWAT deploying this patch as is - as it may be masking a bigger symptom.

Change 273711 abandoned by Jdlrobson:
Don't use decodeURIComponent on Undefined

Reason:
Will roll out on train today.
We may need to SWAT a follow up patch though

https://gerrit.wikimedia.org/r/273711