This seems to be working now, although the earlier problem was probably due to the energo server limiting the number of requests that PVOutput is allowed to make.
Thanks BB much appreciated!! 
This system is receiving a 401 authorization error from the Energomonitor server meaning the username/password is incorrect. The username is typically an email address.
There appears to be a problem with the terminology: The PVOuput form asks for the Energomonitor “Username”, which is “kjahn1” - this is what I use every day to log in to their website. Energomonitor also has a “registered email address”. I have entered this now as “Username” in the PVOuput form, but still no luck. Also re-typed the password, just in case, with no change.
Anyway, being short of detailed logs from PVOutput, I appreciate your help.
The updated details using email is still returning unauthorised.
Energomonitor support should be able to help support@energomonitor.cz
Problem solved. It wasn’t the email address vs. username issue, they want the “username”, but this recommendation fixed it:
“change your password to something really trivial and try one more time”
Slightly different rules for interpreting English vs. Czech character sets? …
Thanks for your assistance.