LOAD DATA LOCAL INFILE failed with error "Invalid utf8 character string"

Dear community,

We recently patched our database server from 5.5.29-30 to 5.5.51-38. After the patch, we noticed that we could no longer use LOAD DATA LOCAL INFILE due to error “ERROR 1300 (HY000): Invalid utf8 character string”.

During our troubleshooting effort, we have tried the following changes:
[LIST]
[]Verify all character set related variables: show variables like ‘%char%’;
[
]Verify client character set
[]Verify source data type
[
]Verify schema DEFAULT_CHARACTER_SET_NAME and DEFAULT_COLLATION_NAME
[*]Adding “CHARACTER SET utf8” to LOAD DATA LOCAL INFILE
[/LIST]
None of these steps helped to resolve the issue. We eventually had to roll back to the old version and managed to use LOAD DATA LOCAL INFILE again (on the same table with the same data file).

I appreciate if the community could shed some light on the potential causes of this issue.

Thanks and regards,