The answer seems to be:
"If your VPN server is running OpenVPN 2.3 or earlier, and the client configuration was designed for OpenVPN 2.3 clients, in some instances you may encounter connection problems when using modern versions of Viscosity (which ship with OpenVPN 2.4) due to a mismatch of VPN compression settings. OpenVPN 2.4 introduced a number of new compression options, and also enforces stricter requirements between the settings on the server and client.
A mismatch of VPN compression settings can result in a warning of "Bad compression stub decompression header byte" appearing in the connection log. However in many instances, depending on the compression settings, it can result in a non-functional VPN connection with no warnings at all.
OpenVPN 2.3 was far more lenient about compression configuration mismatches between client and server. You may have wanted your connection to use compression, but if there is a configuration mismatch, OpenVPN 2.3 would generally not show any errors or warnings, instead compression would simply not be used. OpenVPN 2.4 however will drop packets using a different compression setting than expected. This can result in a VPN connection establishing, however it being essentially unusable. "
I am getting a new error connecting to my VPN. It looks like the OpenVPN needs to be updated to 2.4 version maybe?
"Bad compression stub decompression header byte: 42"
The answer seems to be: "If your VPN server is running OpenVPN 2.3 or earlier, and the client configuration was designed for OpenVPN 2.3 clients, in some instances you may encounter connection problems when using modern versions of Viscosity (which ship with OpenVPN 2.4) due to a mismatch of VPN compression settings. OpenVPN 2.4 introduced a number of new compression options, and also enforces stricter requirements between the settings on the server and client.
A mismatch of VPN compression settings can result in a warning of "Bad compression stub decompression header byte" appearing in the connection log. However in many instances, depending on the compression settings, it can result in a non-functional VPN connection with no warnings at all.
OpenVPN 2.3 was far more lenient about compression configuration mismatches between client and server. You may have wanted your connection to use compression, but if there is a configuration mismatch, OpenVPN 2.3 would generally not show any errors or warnings, instead compression would simply not be used. OpenVPN 2.4 however will drop packets using a different compression setting than expected. This can result in a VPN connection establishing, however it being essentially unusable. "