VNC connection fails with "Value cannot be null. (Parameter 'stream')" on 1.78.2 Nightly Build 3405

Summary

I'm reporting a reproducible issue affecting VNC connections in mRemoteNG Nightly Build 3405.

The same x11vnc server works correctly with the current Stable release.

Environment

• mRemoteNG 1.78.2 Nightly Build 3405 (x64)
• Windows 11 25H2
• x11vnc 0.9.16
• Protocol: RFB 3.8
• Security type: VNC Authentication (type 2)

Reproduction Steps

1. Run mRemoteNG 1.78.2 Nightly Build 3405 x64.

2. Create or open a VNC connection to an x11vnc server using RFB 3.8 and VNC Authentication.

3. Start the connection.

4. The connection fails with:

   Value cannot be null. (Parameter 'stream')

5. Repeat the same connection using mRemoteNG Stable 1.76.20.

6. The connection opens normally.

Error

Opening connection failed!
Value cannot be null. (Parameter 'stream')

Validation performed

Before investigating the application itself, I verified:

• TCP connection to port 5900 succeeds.
• The server returns RFB 003.008.
• The server advertises VNC Authentication (security type 2).
• x11vnc completes protocol negotiation correctly.
• The same server connects normally using mRemoteNG Stable 1.76.20.

Because of this, the issue appears to be specific to the Nightly implementation.

Comparison with Stable

The same connection parameters, server, credentials and network environment work correctly with mRemoteNG Stable 1.76.20.

The issue only occurs with Nightly Build 3405.

Investigation

To validate a possible cause, I downloaded the VncSharpCore source code, modified the RemoteDesktop initialization to safely handle a missing embedded resource stream, rebuilt the project, and replaced the original VncSharpCore.dll included with mRemoteNG Nightly Build 3405.

With this rebuilt assembly, the VNC connection opened and worked correctly using the same server, credentials and configuration.

This suggests the issue is related to the current initialization path in VncSharpCore, although I cannot confirm whether this is the root cause or simply a condition that triggers the failure.

If helpful, I can also provide:

• x11vnc server logs
• mRemoteNG debug logs
• the local source code changes used to rebuild VncSharpCore

Additional information

This may be related to the following discussion, where other users reported the same error:

Possibly related to mRemoteNG Discussion #2696.

Start with VncSharpCore's RemoteDesktop initialization and the embedded resource stream handling described in the report. Reproduce the failure against x11vnc using RFB 3.8 and VNC Authentication, then compare Nightly Build 3405 with Stable 1.76.20. Done means the same VNC connection succeeds without replacing the bundled assembly.