I've found out that the slider script indeed supports decimal filters, and I have already implented it for the upcoming version based on your request, which will be available within 48 hours after testing.
If you have any kind of online test environment I gladly take a look and try to implement a temporary solution to allow decimal filters.
The error message unfortunately does not reveal the source of the issue, as it indicates a parsing error for a JSON structure, can be caused by anything, it's impossible to tell without actually seeing it. From what I can tell there is something wrong with the JSON data that is parsed by the plugin.
Since there was a missing ctype library error before, it might prevented loading all the default options, and some of them might be missing for this search instance. Try deleting this search instance and creating a new one instead to make sure the data is in sync with the defaults. If that is the problem, this should solve it.
If I may note: I've used AMPPS local server for a good bit, but I had to switch to XAMPP and eventually Vagrant powered test environments. AMPPS is great, but I had the same issues – missing libraries, strange configurations for PHP, MySql – and it became very different from an average hosting environment in terms of configuration, and migrating to the live environment became a nightmare because of the errors. I've written an article about transitioning to Vagrant. I'm using this configuration, which is a standard ubuntu LAMP installations, which most servers use nowadays.
I can also suggest digital ocean as they have very cheap hosting packages (5$/month, and it's very fast) for testing purposes. It's unmanaged VPS, much harder to configure, but now they have a one click wordpress package as well – which for testing purposes is more than enough.
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