photopress vs. permalinks :-/

I had this very similar error.
What you should do is:
Go to the photopress config, and enable permalinks
then go to the permalinks configuration and reconfigure it, without making any changes so the .htaccess file is updated, adding the new rules for photopress.

It worked for me.

If you still have problems, check that your .htaccess files is chmoded to 666.

For more info see:
https://codex.www.ads-software.com/Using_Permalinks