Word with brackets triggers 404 error upon save

First time ever received ‘page not found 404 error’ when clicking ‘save draft’, ‘publish’ (from new post) or ‘update’ (if i had already published post without bracketted word). Traced back my steps, deleted 1 word in brackets and error no longer appeared. Funny thing is, i have another word, same post, with brackets and that doesn’t trigger an error message? I’m sure it’s that because i’ve tested inserting and removing the word and it happens as above every time.