Resolved.
Found the thread where the same issue popped up with Patch 7.2.
It seems 2 windows opens named "World of Warcraft" when you launch the game, where 1 of the windows are hidden, invisible or what ever.
This especially seems to be supported by the fact that a renaming command needs to be executed twice, b4 the gaming window is renamed.
I added this renaming command to my script:

<Hotkey ScrollLockOn Ctrl R>
<SendPC local>
<RenameWin "World of Warcraft" WoW1>
<RenameWin "World of Warcraft" WoWa>
<SendPC "PC 2 IP address">
<RenameWin "World of Warcraft" WoW2>
<RenameWin "World of Warcraft" WoWb>

This way the hidden windows get named WoW1 on Server PC and WoW2 on the Client PC.
The gaming windows are renamed Wowa on Server PC and WoWb on Client PC. I didnt bother renaming them again, it's
not needed, I just send the keystrokes to WoWa and WoWb. It works again.
Thanks to muldorante for identifying the core issue.

Yours truely