OK. After playing around some more I found a solution.
The user I was using on this device was a local one which causes the prompt of this device to end with a $-character.
On the other devices I use remote users (TACACS) which causes the prompt of those devices to end with a #-character.
As soon as I changed the user to remote the download went smoothly.
I had added
<Command Name="MenuBased" Value="false" />
<Command Name="VirtualPrompt" Value="\$ " />
to the device template in question to support the non-standard prompt. Could it be that this caused the problem? I had tried out several things, even the
<Command Name="UseExactPromptMatch " Value="true"/>
<Command Name="VirtualPrompt" Value="8400a\-p\-afw001\ \$ " />
but it always ended in the same error.