Rather than defining an additional enum I would use the following sequence in the SoDebugError header and source files:

#if defined(_WIN32)
#pragma push_macro("ERROR")
#undef ERROR
#endif

< code that defines enum in header and that uses enum in source >

#if defined(_WIN32)
#pragma pop_macro("ERROR")
#endif

This will save the value of the ERROR macro on the stack with the push_macro command, undef the macro in the current context and restore its previous value after the pop_macro command.
Although push_macro and pop_macro are Microsoft preprocessor extensions (defined at least since VS6 from 1998) they are also widely supported from in Intel C++, GCC and Clang compilers.

It looks good, but SoDebugError::ERROR will become an error then?

Not in SoDebugError.h and SoDebugError.cpp, the ERROR preprocessor token is undefined and a new enum constant SoDebugError::ERROR can be defined without conflicts. But in other sources that eval SoDebugError::ERROR and include windows.h you need to insert the same sequence. A quick grep in the entire Coin3d repository revealed only one place in SoWin.cpp where SoDebugError::ERROR is used. This push_macro/pop_macro is used for instance in Google protobuf (see here )

Shall I do the necessary changes to the files or will you adapt the PR?