After manually updating to 5.3.74, I had the same error, deleted /bootstrap/cache, that got me back.
Now, I see 5.3.75 as an available update, and I get a server error trying to get the update.
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deleted the bootstrap cache after manually updating and everything is working. Hopefully the auto update will work next round, as that has been convenient!
This worked perfectly. Thanks @gene Someone should turn this into a installing script 
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