Tried copying the BouncyCastle.Crypto.dll from the previous RPM file and the message has now stopped appearing for any jobs, so it does seem something needs fixing in MimeKit like you thought.
I do however get this which I think was logging before:
{"ClassName":"System.Net.WebException","Message":"Error: TrustFailure (Authentication failed, see inner
exception.)","Data":null,"InnerException":{"ClassName":"System.Security.Authentication.AuthenticationException","Message":"Authentication failed, see inner
exception.","Data":null,"InnerException":{"Message":"Ssl error:1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED\n at
/build/mono-2liaiA/mono-5.18.0.240+dfsg/external/boringssl/ssl/handshake_client.c:1132","Data":{},"InnerException":null,"StackTrace":" at
Mono.Btls.MonoBtlsContext.ProcessHandshake () [0x00054] in <a9a08e39ba304bd0a84c49bd158dfc02>:0 \n at Mono.Net.Security.MobileAuthenticatedStream.ProcessHandshake
(Mono.Net.Security.AsyncOperationStatus status, System.Boolean renegotiate) [0x000c6] in <a9a08e39ba304bd0a84c49bd158dfc02>:0 \n at (wrapper
remoting-invoke-with-check) Mono.Net.Security.MobileAuthenticatedStream.ProcessHandshake(Mono.Net.Security.AsyncOperationStatus,bool)\n at
Mono.Net.Security.AsyncHandshakeRequest.Run (Mono.Net.Security.AsyncOperationStatus status) [0x00006] in <a9a08e39ba304bd0a84c49bd158dfc02>:0 \n at
Mono.Net.Security.AsyncProtocolRequest.ProcessOperation (System.Threading.CancellationToken cancellationToken) [0x0012a] in <a9a08e39ba304bd0a84c49bd158dfc02>:0 \n
at
Mono.Net.Security.AsyncProtocolRequest.StartOperation (System.Threading.CancellationToken cancellationToken) [0x000a4] in <a9a08e39ba304bd0a84c49bd158dfc02>:0
","HelpLink":null,"Source":"mscorlib","HResult":-2146233088},"HelpURL":null,"StackTraceString":" at
Mono.Net.Security.MobileAuthenticatedStream.ProcessAuthentication
(System.Boolean runSynchronously, Mono.Net.Security.MonoSslAuthenticationOptions options, System.Threading.CancellationToken cancellationToken) [0x00336] in
<a9a08e39ba304bd0a84c49bd158dfc02>:0 \n at Mono.Net.Security.MonoTlsStream.CreateStream (System.Net.WebConnectionTunnel tunnel, System.Threading.CancellationToken
cancellationToken) [0x0018c] in <a9a08e39ba304bd0a84c49bd158dfc02>:0 \n at System.Net.WebConnection.CreateStream (System.Net.WebOperation operation, System.Boolean
reused, System.Threading.CancellationToken cancellationToken) [0x001f5] in <a9a08e39ba304bd0a84c49bd158dfc02>:0
","RemoteStackTraceString":null,"RemoteStackIndex":0,"ExceptionMethod":null,"HResult":-2146233087,"Source":"mscorlib"},"HelpURL":null,"StackTraceString":" at
System.Net.WebConnection.CreateStream (System.Net.WebOperation operation, System.Boolean reused, System.Threading.CancellationToken cancellationToken) [0x00275] in
<a9a08e39ba304bd0a84c49bd158dfc02>:0 \n at System.Net.WebConnection.InitConnection (System.Net.WebOperation operation, System.Threading.CancellationToken
cancellationToken) [0x0015b] in <a9a08e39ba304bd0a84c49bd158dfc02>:0 \n at System.Net.WebOperation.Run () [0x000b7] in <a9a08e39ba304bd0a84c49bd158dfc02>:0 \n at
System.Net.WebCompletionSource`1[T].WaitForCompletion () [0x000b1] in <a9a08e39ba304bd0a84c49bd158dfc02>:0 \n at System.Net.HttpWebRequest.RunWithTimeoutWorker[T]
(System.Threading.Tasks.Task`1[TResult] workerTask, System.Int32 timeout, System.Action abort, System.Func`1[TResult] aborted,
System.Threading.CancellationTokenSource
cts) [0x00118] in <a9a08e39ba304bd0a84c49bd158dfc02>:0 \n at Duplicati.Library.Utility.AsyncHttpRequest+AsyncWrapper.GetResponseOrStream () [0x0004d] in
<d644d641ef094b6cbc223bc3a32d53af>:0 \n at Duplicati.Library.Utility.AsyncHttpRequest.GetRequestStream (System.Int64 contentlength) [0x00068] in
<d644d641ef094b6cbc223bc3a32d53af>:0 \n at Duplicati.Library.UsageReporter.ReportSetUploader+<>c.<Run>b__3_0 (CoCoL.IReadChannel`1[T] chan) [0x000c4] in
<5d01de97f88c47219817ca3929a597f5>:0
","RemoteStackTraceString":null,"RemoteStackIndex":0,"ExceptionMethod":null,"HResult":-2146233079,"Source":"Duplicati.Library.Utility"}
With regards to NFS, I assumed that using an SMB share like this meant that it was supported which is why I asked if I could do the same with an NFS share
BTW, I think I might have found the NFS file issue which could be a permissions problems with the files themselves, so I have reset the owner/rights on all share and will see how it goes. How they got messed up is beyond me.