Huge spherical panorama to cubic - I can't save it

Why it is not possible to change big spherical panorama 6500x13000 or 8000x16000 to cubic panorama and save it? I use new PW8.6. I import big spherical panorama, I convert it to cubic but I can't save it.

Are you using standard or professional version? Windows or Mac? Please send the images to easypano.jack@gmail.com via www.wetransfer.com. If I can convert the image successfully, I will send back a cubic panorama.

It's some bug in PW Pro 8.6 . I can't save big cubic panorama larger than 6000x12000 in any format. I sent panorama to email.

Windows 7 64bit, 16gb ram, Intel i5, SSD.