Thanks for your report. I have fixed this. I'm sending you a development build.
When browsing to my cert *.key, one folder has a space in it. After selecting the key, the full path is added to the GUI with a pair of quotes around it (Looks like: "C:\test folder\key.key"). When connecting to the host, I get the error:
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Error
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Cannot open certificate ""C:\test folder\key.key"".
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System Error. Code: 2.
The system cannot find the file specified
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OK Reconnect Help
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Resolution is to remove the pair of quotes that are in the parameter box before saving the connection.
This is using v5.8 built 2 days ago. Please let me know if you need any additional description. Thanks for all your work!