If 5.6 runs and 7 not> normally influence of a script or plugin (older ones), theme MAY happen too.
Personally i think it is a plugin, not belonging to core. The easiest way to find the cause is to take a look in the server logs, as they give a clue where something goes wrong (although message might be cryptic).
Try if possible to use Bootstrap 3 for installment (theme can be switched afterwards).
I do advice to upgrading on a local base though (backups at hand), and you did not point out : are you working locally? or live? If local> error can be on local server (inactive modules as php 7 needs more than standard). Is it live ? ...
Can you list your plugins in use?