The ARC reading code cannot (or at least does not) recover and the exception unwinds the stack, aborting the reading of the ARC file.
In the file FS-359228.arc.gz, there are records which have a header line with a mime-type containing a space, such as:
Notice the mime-type field is "Mozilla/2.02Gold (Win95".
The ARCRecord.computeMetaData() method is passed the list of values, which it assumes adheres to a fixed set of fields. It checks to see if the number of values matches the number of expected fields, and if not it does apply a bit of guessing to see if it can fix things.
It appears that the code was tweaked to handle one particular case where a space appears in the mime-type, but it doesn't handle all cases, the code is:
so it looks like someone added a specific check for a case where a mime-type starting with "charset=" had a space in it.