UK 1994 Ford Escort - Central unlocking

Since yesterday I cannot lock my Ford Escort.
When I turn either of the front door locks it locks then the motors sound like they're struggling, then it unlocks again.
It is a five door 1.6 LX. Have tried locking over 100 times and it fails each time and unlocks all doors.
The only way I can lock it is by disconnecting the battery and locking each door one by one!
Any ideas on this please?
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