TOPIC:

Rendering Error in layout Message/Item: array_keys(): Argument #1 ($array) must be of type array, null given. Please enable debug mode for more information.

Are you sure it is "openhighlight.grf" and not "innerhighlight.grf"?

If it is the later, in the gameclient page, below the "Source files" image, there is a link where you can donwload "innerhighlight.grf". Then you copy it to ~/.openttd/baseset

It should work.

Sorry for the confusion, I did mean innerhighlight.grf and it is in the right location. Otherwise the client wouldn't run when starting from its folder from the command line.

edited the original post