Re: create account or register – 500 error
500 is an internal server error or misconfiguration it might be caused by to low a php script memory value judging by the sound of the profile field page error.
As always to troubleshoot first disable BP confirm that WP works as expected without issue, in this case that you can register accounts without issue, when that is confirmed as ok proceed to enable/activate BP but no other plugins just plain vanilla BP along with the default theme NOT a custom one, confirm whether issues arise.
You might possibly have syntax errors in .htaccess but if so I would expect issues from the get go as it’s read on page load .
If possible confirm the amount set for PHP script memory in php.ini.
