I had similar on a VAX 11/750. The LA120 DECwriter was connected to the system via a serial port mux adapter. One night the mux adapter glitched and reset itse

I had similar on a VAX 11/750. The LA120 DECwriter was connected to the system via a serial port mux adapter. One night the mux adapter glitched and reset itself back to defaults, which turned on local echo, so it reflected 'login:' back to the VAX, which responded with the password prompt. This prompt was reflected too, giving an incorrect password response...which was reflected back to a new login prompt...etc..rinse, repeat all night. Everything was printed on the LA120 until the fanfold paper ran out. I came in the next day and could not see the LA120 due to the amount of paper on top of it.