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Is it disabled on other extra path options too? Any javascript errors?

Oddly, it is fine for "category prefix".The problematic one is "tags prefix".I checked under both blog , and seo settings, it s disabled on both pages.
No javascript errors.
I simply get around by hacking into html and removing disabled="disabled".But if i forget to do it each time i change something, it goes back to default "/tag" prefix

Is it in v5? Sounds like a bug.

Nope, 4.1.3.Havent gotten to upgrade yet.
@sam2kb wrote:

Is it in v5? Sounds like a bug.

then try at least v4.1.6