This is the offending line, an OSBYTE call, which I've broken down to make it more comprehensible. What it's doing is *FX143,18,5 and something doesn't like it.
OSBYTE 143 issues a ROM service call, 18 is the service call for select filing system, and 5 is the filing system number (not the filing system ROM number), with 5 being the Econet filing system.
Basically, it's Acorn's way of changing filing systems, which they use in many utilities. Using *NET would fail if there is no clock signal detected and the filing system doesn't change to NFS so you remain in the previous filing system, which could cause havoc with the utility if it's expecting a filing system change which hasn't happened.
Instead Acorn use the service call 18 method. It forces selection of NFS, or ANFS when on a Master, and works in all cases - except here of course!
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