A 500 error in WordPress can be caused by several things (it’s a general error message). The most likely culprit is not having enough memory allocated to WordPress – search for “how to increase the WordPress PHP memory limit.” If that doesn’t help, search for “how to enable WordPress debugging” so you can see more information about what is happening.
If you do see more information about what is happening, and it seems to be a problem specifically with NarraFirma, please let us know by posting an issue on the NarraFirma Github site.