Why ngspice doesn't accept UIC for TRAN?
NGSPICE version 20 is much too old. Please upgrade to version 26.
The current version has no problem with UIC.

PS: "ngspice 1 -> tran 10m 5 UIC" should probably be
"ngspice 1 -> tran 10m 5 0 10ms UIC" (parameters are extracted by position).