This problem is caused by a Net8 / Net9 installation issue. PL/SQL Developer attempts to load OCI.DLL (the Oracle Net library), which subsequently cannot find on of its required modules. This is usually a PATH related problem.
Can you go to Help > About, press the 'Info' button, press the 'Copy all pages' button on this info screen, and paste this text into an e-mail reply? This should allow me to provide a solution.
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Marco Kalter
Allround Automations