Arduino portable crashes with SignatureVerificationFailedException

[daverobo]

Arduino IDE 1.6.8 + portable installation + ESP8266 2.3.0 platform package.
Works OK on original host (Windows 7 SP1 64-bit).
Crashes when copied to another host (also Windows 7 SP1 64-bit).
Output from arduino_debug.exe:

Loading configuration...
Initializing packages...
cc.arduino.contributions.SignatureVerificationFailedException: package_index.json file signature verification failed
        at cc.arduino.contributions.packages.ContributionsIndexer.parseIndex(ContributionsIndexer.java:82)
        at processing.app.BaseNoGui.initPackages(BaseNoGui.java:597)
        at processing.app.Base.<init>(Base.java:295)
        at processing.app.Base.guardedMain(Base.java:222)
        at processing.app.Base.main(Base.java:137)

File "lib\public.gpg.key" exists.
File "portable\package_index.json" exists.
File "portable\package_index.json.sig" exists.
Date Modified: timestamp(package_index.json.sig) > timestamp(package_index.json).

Seems to be failing in the GPGDetachedSignatureVerifier.verify() method which is called by the SignatureVerifier.isSigned() method. This suggests that the PGP fingerprint does not match but I don't know why. All files including the key/json/sig files have the same contents on both hosts.