1668 might not able to upgrade directly to 1941, according to TPI it should be at least 1788 or higher. Also on 1941 update files there are exclusion for below version of software:
"BlockedTrains" : [
"MOI3_EU_SK_R5890L",
"MOI3_EU_SK_R5910L",
"MOI3_EU_SK_R6030L",
"MOI3_EU_SK_R6040L",
"MOI3_EU_SK_R6210L",
"MOI3_EU_SK_R6400L",
"MOI3_EU_SK_R6460L",
"MOI3_EU_SK_R6500L",
"MOI3_EU_SK_R7160L",
"MOI3_EU_SK_R7170L",
"MOI3_EU_SK_R7290L",
"MOI3_EU_SK_R7300L",
"MOI3_EU_SK_R7450L",
"MOI3_EU_SK_R7550L",
"MOI3_EU_SK_R7560L",
"MOI3_EU_SK_R7570L",
"MOI3_EU_SK_R7580L",
"MOI3_EU_SK_R7590L",
"MOI3_EU_SK_R7600L",
"MOI3_EU_SK_R7610L"
],
Which means you need first upgrade to 1788 or higher and then do 1941.