if you restore the db, you need to restore the old version too…  the database version cannot be newer than the version of the software… 

this is because the db has already been upgraded to any new db schema of the new version…  going to old version, may cause significant problems…  so its disallowed…

you can check your db version in the sfoptions table, option  key of sfversion…

you can check your code version in wp-content/plugins/simple-press/sp-control.php

My db version is 5.5.3 and the code version is 5.5.1. Do I need to change the version on the server to 5.5.1

No, you need to put the 5.5.3 code version in your wp plugins folder…  it needs to match the db schema/version…   code version 5.5.1 is several versions old… 

If you had a db backup from when you were running 5.5.1, you could restore it…. but would use caution on this…

Can you explain how I do this. Where do I get the get the 5.5.3 version and where in the folder to I put it?

just go to the wordpress dashboard updates… you should see simple press update listed there…  its where wp updates itself, plugins and themes…

alternately, you could download it from our download page and upload via ftp to your wp plugins directory…

the former is easier…

Thank you so much for your help, all good now.

Great news. Thanks for the update.