It puzzled me so much when my mail receiver didn’t work anymore randomly.
I could trace the error back to my last rebuild, and noticed the API username I set in the config file didn’t exist anymore on my forum.
I had indeed changed the system account’s username, and it was reverted after the rebuild.
That’s how I learned we couldn’t, in fact, change this account username.
I’d prefer that we just couldn’t change it at all in the interface.