We get this error today, and can't update any query.
I am experiencing the same issue.
It seems that today a lot people experiencing the same error, the ERROR 400 PLACEHOLDER.
I haven't got answer from the team.
We're also seeing this error sporadically. Random queries from same data source (AdWords MCC, for example) work and some fail with above error.
Just retried our queries and they all went through fine this time!