TCL error :Prerequisite operation not in progress (line ...

It did not do the trick, however after some extra testing I was able to avoid the TCL error. The solution was to disable HTTP:class before disabling HTTP. The order seems real critical. The rule looks now like below.

when HTTP_REQUEST {
    if {[HTTP::uri] starts_with "/partinsert"} {
         if { [string tolower [HTTP::header Upgrade]] contains "websocket" }{
           log local0. "websocket detected"
            HTTP::class disable
            HTTP::disable  
            }  
        }
    else {
        HTTP::uri "/partinsert[HTTP::uri]"
        }   
    }

Thank you all for helping me to get to this solution. Should you have remarks on the last solution, let me know.