Peter,
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I followed the steps in your first part, clicked on "Import Decoder File", selected my file and clicked open. In 5.10, I waited and waited, but nothing happened after that. In 4.26 all completed without an issue with a dialogue box popping up saying it was completed. I then restarted DecoderPro and my file was in the Decoder list as expected.
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Perhaps my Windows 10 PC is not sufficiently fast enough to cope with version 5.10. Just to be clear I installed Azul Java 17 before installing 5.10.
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So not sure why I couldn't add a new Decoder when you clearly could!
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Regards?
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Roger