Hi, we encounter such error while test calling backend service from APIM management plane and already found a workaround.
The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.",
"The remote certificate is invalid according to the validation procedure
Workaround :
Disable Validate certificate chain and Validate certificate name from Backends > Properties , the error is gone.
One question here
Is the workaround safe ? Azure resource shows http in protocol which concerns us if this way uses insecure HTTP instead of HTTPS for data transmission.

Need your advice, thank you