I finally have a working 64bit version of Euler. It turned out to be more difficult than expected to transfer the old code to something, which would work for 64-bit architectures. As an example, some parameters to the window API were „long“, which surprisingly is 32-bit even in 64-bit C. I had to change the types to „LPARAM“, which I should have done to start with. Besides, I sometimes used long to store pointers, which does not work by the same reason.
Moreover, I had to compile EULPSOLVE for a 64-bit version, and get a 64-bit version of Tiny C to work with Euler. Finally, the Inno updater needed an update to support 64-bit installations.
I gave up on version numbers, and use the date of publishing simply. The new version is Version 2013-11-11.