NetBeans - ERROR: Language-Definition not correct!

[HansJansen]

Hello,
I am completely new on this site, so my apologies in advance if I do things I shouldn't!
I am a retired software developer (since 2002...) trying to refresh my Java skills. I acquired source code of a very old program and created a NetBeans project for it, as "project with existing source".
After correcting a number of deprecating constructs I could compile the whole stuff without errors (only a number of warnings which I intend to get rid of later).
When I try to run the result, I get the following error:

ERROR: Language-Definition not correct!

... and the process is stopped without even starting a log file.
I cannot find anything about this error in NetBeans documentation, nor anywhere else on Netbeans, Apache or several Java forums.
Can anybody here shed any light on this problem for me?

Thanks in advance,

Hans.

[dev7060]

When I try to run the result, I get the following error:

ERROR: Language-Definition not correct!

How can one reproduce the same?

I acquired source code of a very old program and created a NetBeans project for it, as "project with existing source".

Did you try running without an IDE? What file(s) does the 'old program' have? Did you try setting up a new project?