iDRAC FWUpdate with racadm fails

HI all,

some specs first:

iDRAC8, Current Firmware: V2.10.10.10

RACADM ,V8.2, x64

I try to update the iDRAC Firmware with V2.21.21.21 and the racadm remote connection.

Syntax:

racadm.exe -r 1.2.3.4 -u MyUser -p MyPassword fwupdate -u -p -d FW_V2.21.21.21.exe

After some time I get the error message:

ERROR: Unable to transfer image file to RAC.

Most the time, next try with same syntax will give:

ERROR: Remote host is not reachable or connection is interrupted.

Also there is a firewall between the racadm.exe host and the iDRAC.

Only TCP/443 and TCP/5900 are allowed.

Is there any other port necessary?

Is there any user/admin rights missing?

Wich interfaces are required for this type of FW update? (Telnet, SSH, IPMI,....)

Any help/info is much appreciated!

Thanks!