I had this problem of Google preferences failing to save.
All had been well for the last month or so on my new computer where I had ben able to save preferences without any issue.
Then in the last couple of weeks I began to delete cache and cookies (as a bit of a space saving and housekeeping thought) and shortly after this it seemed Google suddenly stopped allowing me to save preferences and the safe search off would always divert back to moderate search.
I read up on possible solutions and nothing I found seemed to solve the issue.
Last night I shut off my modem and then turned it back on (effectively resetting it) this morning I can modify search preferences in Google so this may have solved things.
Hope this helps.