A quick Google Search: "code 91 salt cell" tells me OP has an Intex 8110. The search turned up many suggestions. In a nut shell top of the list is:

1. Scaling can give a false low salt reading, needs to be cleaned - You did this already so check.

2. Cell could be going bad - How old is it?

3. It's possible you really are low on salt. Have you done a manual test?
 
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