Symptoms

The client performed some configuration on the SourceCode.Forms.Runtime.config file and in an attempt to "fast track" things, copied the config fir to 3 other servers. This caused the "unique" server cipher keys to be overwritten with the key from the 1st server. This resulted in 3 of the 4 servers being inoperable.
 

Diagnoses

The cypher key on each server should be unique. This key is made up of various pieces of information that is specific to that one machine only. The server will fail without its unique cipher key.
 

Resolution

We replaced the "faulty" cipher keys in the SourceCode.Forms.Runtime.config file with the "commented section" directly above it. After that, we restarted the K2Server machine and the cipher got re-created again. You have to do this on each machine with a invalid cipher key.