If you're in the EU, you could use EU Directive 2009/24/EC. Someone with a better grasp of the law should check this, but you should be allowed to decompile & fix parts of the program which causes "interoperability" problems. It doesn't matter what EULA says or if it's copyrighted code.

EDIT: appartently there was a ruling in the Court of Justice of European Union on this

https://www.lexology.com/library/detail.aspx?g=f5b1193c-f423...