The standard way to debug such errors in Koha are as follows

1) Completely clear browser cache and retry

2) If #1 does not work and error persists, check your browser console for errors

3) If #2 does not show anything OR does not show anything that is relevant to the error you are encountering, then tail the intranet-error.log and see what is logged when the error occurs on the GUI. Typically, the log file will show the actual Koha perl script or module which is throwing the error along with the line number. Then one should open that file, go to that line number and by reading the code figure out what is missing and cross check against the data in the database.

thanks pretty much the basics of debugging Koha.

hope this helps