Recently links with query parameters on my wordpress website stopped working. They now just return blanks pages. However the same links without query parameters works fine. Example: https://www.example.com/signup/?Subscription=3 - returns blank page https://www.example.com/signup - works fine
What can be the problem and what are the ways to solve it?
As a quick check, make sure you refresh your permalinks by visiting Settings > Reading > Permalinks in your control panel.
If this doesn't help, see below.
I'd start by doing standard WordPress diagnostics. That is, disable all plugins. If it doesn't work still, try switching to a default theme.
If it still doesn't work after that, I would say that it is either a problem with your .htaccess file if it has been modified (by yourself or a plugin), or it's a problem with your web stack. If you're sure it's neither, then it may be a faulty WordPress install, or a bug within the core.
If it breaks without plugins but works when you swap themes, it's a problem with your theme.
If it works without plugins without switching themes, it's a problem with a plugin. Try testing it by enabling them one at a time and attempting to repeat the issue.
Without more information / context, there's not a lot more I can do to help isolate where the problem is.