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Tried connecting using Sonicwall Mobile Connect v.5.0.7 on Mac OS Big Sur v11.4, it shows Invalid Server Certificate then immediately says disconnected
Issue resolved, infected by malware.
Is the certificate on the SSLVPN device you are connecting to a self signed cert?
BigSur is very picky about certs. When you try to connect if you look at the cert details on the Mac and tell it to trust that cert, it will likely clear the issue.
Alternately you can get a CA signed cert.