Hi Juan,
It seems to be failed because of lower case letters. You can change it to upper case in PSA(as told above) for the time being. You can do this if the number of records containing this value in PSA is less. For safe side prepare one conversion routine to make character values to upper case and put it in Data source(but in most support scenarios it is not a simple process to add this routine to Data source at the time it failed because of restricted access of modification in production environment).
If you found it difficult to search for lower case letters in PSA then you can follow the below methods just for a temporary solution:
1. You can change(if changing dtp properties are allowed in prod) the Error handling process of DTP to "Update valid records, reporting possible" and can get the list of small case values in the error stack and modify them.
2. If changing to DTP is not allowed in prod then - Download the PSA content in ms excel, set auto filter in the required columns(of excel) then from the drop down you can find any small case letters present. You can list down the values and put them in PSA filter and change them to upper case(this is a hectic process but if no option available and you have to load the values then this can be a life saver).
Thanks.