You appear to have an object placed that is causing this error. The flag is referring to a portion of GDL code that tells an object that it is curved, or is not curved.

You probably placed an object or change the parameters of an object that was not completely error proofed in it's coding. Try marqueeing half of the project and viewing it in 3D. If you get the error then the bad part(s) are present in what you see. If not select the other half of the project. Continue selecting halves of halves until you identify the problem part.

If the error message gives you an object ID then you can also isolate all objects with that ID and repeat the above process with a much smaller group.

HTH