Have you tried manually downloading the package from outside of OpenTTD's internal online content downloader and checking that it works there? This could be related to #6969.

Curious, but seems very unlikely to be (directly) related to #6969

I'm trying to trace it

I think it's because

require("../enginemanager.nut");
require("../utils.nut");
require("../investment.nut");
require("../industry.nut");
require("../town.nut");

Can you elaborate?

The "cannot open the file" error is thrown when trying to open "notperfectai-1.0\transportplan\..\enginemanager.nut" because

Is the file actually present in the tar and just not being detected by OpenTTD's code?

Based on glx's comment I understand the issue to be that OpenTTD is looking for a file named notperfectai-1.0\transportplan\..\enginemanager.nut but the tar contains a file named notperfectai-1.0\enginemanager.nut - i.e. the issue is that OpenTTD is not resolving relative paths by collapsing the subdirectory and up-directory in the path to nothing.
So the solution would be to look for .. path components and have them remove the previous path component.