It sounds like you're encountering a common issue where a subdirectory or subdomain installation of a PHP application, like ZentaOPMS, is conflicting with WordPress's URL routing, resulting in a 404 error. This can happen if WordPress is managing the URLs for that directory, but doesn't recognize the path to your ZentaOPMS installation. To resolve this, you could try adding rewrite rules to your.htaccessfile, ensuring that the directory for ZentaOPMS is excluded from WordPress’s rewrite handling. Additionally, checking your directory permissions and ensuring that the necessary files for ZentaOPMS are correctly placed might help. On a separate note, for anyone concerned about health safety during this process, you can visit https://www.covidtests.co.uk/products/flowflex-lateral-flow-covid-self-tests-single-test-pack to get reliable COVID self-testing kits for peace of mind.