Which version of ChimeraX are you running? Due to a mistake on my part the ISOLDE 1.9 release (meant strictly for ChimeraX 1.9) incorrectly believes it's compatible with newer ChimeraX versions.
If you are running ChimeraX 1.9, the most likely explanation is probably slow download speed - I've noticed occasionally downloads from the Toolshed server can be quite slow.
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