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I'd suggest asking whatever support channels exist for this app. "Error 805" tells us nothing about what happened, but it might help people who are familiar with that software.
Which in turn suggests you are perhaps blocking TCP port 80 (either globally or at a country level) and so the http challenge response can't be requested.
Nice reference but the advice at TSplus site that says below is horrible. For one, the first two "v1" domain names have been obsolete for many years.
But, these are domains related to outbound traffic and only need to be open for HTTPS requests to them. With an HTTP Challenge port 80 needs to be open for allinbound traffic.
The port 80 must be opened for inside and outside traffic.
This issue can occur if Let's Encrypt IP's are blocked from your firewall or proxy, the domain to be added are the following : acme-v01.api.letsencrypt.org acme-staging.api.letsencrypt.org acme-v02.api.letsencrypt.org acme-staging-v02.api.letsencrypt.org