Error upon starting, which persists after reinstalling eMClient backup before re-installing

Ref. the error image below:

  • At first there is the usual rotating circle until a red triangle with exclamation mark, circled in red in the image below, appears next to my account.
  • I cannot send emails.
  • Trying to fix this in ‘Account Settings’ > ‘Diagnostics’ leads to a pop-up to say that ‘Autoconfiguration has failed’.
  • I have checked the username and password, as in eMClient settings, work with the provider’s website and mail.

PS (Re." I would like to try re-installing as others have done but ‘Backup is not accessible’." : I changed the settings to a folder on desktop and could then backup to that file)
PPS I reinstalled eMClient, and ‘File > Restore’ got my data back, but the error message persists…

…Advice would be appreciated.