Rolf —
I found one place on line that referred to another plugin – but the problem is the same.
It talked about doing this — I am guessing this is something the developer needs to do —
Refers to — ‘pluginname/readme.txt’
Inside this file it says,
‘Tested up to: 2.8.4′ or whatever WP version tested to. This number may vary but the important thing is that it shows at least as high as the current version of WordPress that you are currently using.
To fix your “Plugin does not have a valid header” problem, simply edit this number to your current version number or higher and save your changes.
That said, I am suspecting that the version of the plugin I am downloading may not be saying “tested up to 4.0” (the latest version).
The info referred to the fact when this change was made the plugin with the issue ended up working fine.
Does this sound like something that might apply to this situation?