Slightly late to the party on this one but just had the same error on R80.30. Client engaged TAC and ran cpdoctor, apparently didn't find any issues. Tried to check for non-unicode characters but also didn't see anything odd.
Using some of the other posts on here I created the following API command that could be useful for a quick check against policy;
[Expert@SMS01:0]# mgmt_cli show access-rulebase name "POLICY_NAME Network" limit 500 --root true --format json | $CPDIR/jq/jq '.rulebase[].rulebase[] | .["name"], .["comments"]' | -E '^"\s|\s"$'
"Site AP1450 SIC/Policy access "
"Monitoring (Videowalls) to FMS "
I've sanitised some of the output but run on our SMS the output shows several comments or rulenames that have whitespace characters at the end of lines.
I'm still waiting on confirmation that the policy can be cloned now. Will update again if any issues.