@ Carrie
RE: the php memory limit is 256 MB, which should be adequate, no?
Yes, in most conditions, more than enough.
RE: I had upped my php from 5.4 to 5.6 maybe a week prior to this issue starting; I lowered it back to 5.4 after this issue popped up and the error persists.
I would suggest the version that WordPress recommends which is PHP version 7.1. It is far more faster at running WP while being much more resource efficient.
RE: Is there anything else I could do to correct this issue?
Can you look in the PHP error log to see if there are any messages that may help shed light on the issue?
Can you please create a temp admin account using the following plugin so I can take a closer look?
https://wordpress.org/plugins/temporary-login-without-password/
