See:
https://trac.openmodelica.org/OpenModelica/ticket/3318
Get the 64 bit nightly from here:
https://build.openmodelica.org/omc/buil … lds/64bit/

That solved my problem, thanks! Any estimate when the 64 bit version will be official?

We'll be running tests for 2-3 weeks with the new 64/32 bit OpenModelica based on msys2 and mingw64/mingw32 gcc 5.3 and then probably release 1.9.5.
The version which is currently there was manually build by me but it should be  available on in the automatically build nightly-builds soon.

It looks like the 64 bit version is not being built with the nightly builds. Is that as expected?

I'm still fighting to pushing my changes for 64 bit in. I hope to have them in tomorrow and then the builds will have 64 bit too.

Thanks for the update!