If one of the streams scanned as part of a MultiBranch pipeline project contains [] characters in the name, and this stream is not explicitly specifyed as the 'Stream source', the following error is generated:
ERROR: Could not fetch branches from source 5def8e96-7d1e-4c99-b65e-699b7b745540
java.io.IOException: Unable to check file: Illegal character range near index 37

If however the stream containing the [] chars is specifyed as the source, the error does not occur.

P4 and P4D seem able to process the stream name containing [] chars without issues, and I could not reproduce the issue when working with a freestyle job.