'The input is not a valid Base-64 string as it contains a non-base 64 character' error when checking server availability

[JWoodley13]

Describe the bug All uSync servers displaying as unavailable on new environment, config is shared between servers and works everywhere else, nothing is obvious as to what is wrong.

Looking in debug log shows the following error when trying to connect:

{"@t":"2021-07-29T14:44:43.2601981Z","@mt":"Exception contacting {url} : {message}","@l":"Debug","url":"{websiteurl}/backoffice/uSyncReceive/uSyncReceiveApi/Enabled","message":"The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters. ","SourceContext":"uSync.Publisher.Publishers.SyncPublisherBase","ProcessId":9276,"ProcessName":"w3wp","ThreadId":74,"AppDomainId":4,"AppDomainAppId":"LMW3SVC1329640476ROOT","MachineName":"dw1mdwk00000R","Log4NetLevel":"DEBUG","HttpRequestNumber":92,"HttpRequestId":"8c2e18e2-8d51-483e-8e55-5a0aa74e9612"}

Version (please complete the following information):

• Umbraco Version: 8.9.1
• uSync Version: 8.8.3
• uSync.Complete Version: 8.9.0

To Reproduce Steps to reproduce the behavior:

1. Go to Settings > uSync Publisher
2. All servers display as unavailable

Expected behavior All servers display as available as they do on other environments.

[JWoodley13]

Found resolution, issue was overwrites in the web.config file