Hello Azure LoadBalancer expert

Six months ago, I installed a VM in Azure running an MSSQL server. The VM is located in a VNET and does not have a public IP. To access the SQL Server via the Internet, I first installed an external Azure Load Balancer and set up a NAT rule that forwards traffic from the public IP of the LB via port 3378 to port 1433 of the VM. In the NSG, I enabled port 1433 in the VNET (they are all open anyway) and allowed port 3378 to the internal IP of the VM from the Internet.
The port 1433 on VM is open and a connection from another vm in same vnet can be established.

This worked, but then suddenly it stopped working. I probably changed something and can't find the error.

For me looks my setup same like post Azure Load Balancer Inbound NAT rule targeting VM. The only diff is, I do have just one machine in backend pool.

Has anyone an idea, how to solve the issue?

Best Tino