Hello again,
Apologies for the delay in replying, but it took some time to pin this down. Deactivating all plugins and activating them in turn cleared the problem. However, once all were active again the problem re-occurred, as did some other odd behaviors.
Ultimately (although not conclusively) I have tracked the problem down to two plugins: ‘WP Hide & Security Enhancer PRO’ and ‘WP Fastest Cache’.
WP Hide has always been difficult, given the amount or re-writing it does at the front-end. It does also have a functionality similar to your own but I leave it turned off as it still allowed access to ‘login’ as a url, redirecting to the hidden alternative (something I DID NOT want!). For the moment it is still deactivated, pending further investigation.
As for the second one, WP Fastest Cache, I’ve used for some time with no problems (to a point where I have paid for the premium), but of late it has been breaking pages, especially when JS or CSS pages are minified or encoded. I changed to WP Super Cache and now, not only have all problems gone away, but I’ve actually got a much faster site, earning an impressive 93% performance on GTmetrix.
So, my faith has been restored in ‘WPS Hide Login’ and my apologies for the panic. Once again, ‘WPS Hide Login’ is protecting my login / admin pages!
Regards,