Looks similar to
In the test case below:
-value to be matched to in CiviCRM has a period at the end, e.g. "Sept."
-value in cvs field matches a CiviCRM value and also has a period at the end, e.g. "Sept."
-in step 3 of the import, an "Invalid value for field(s): month" is given, field in csv is shown as "Sept"
Reproduced on the demo site with attachment.
-first row contains column headers
2) Load saved field mapping: testbug1
3) You should see 40 errors in step 3.