System error when uploading a new file (step 1: error "moving" a temporary file)

In step 1, when pressing "next", a system error as following appears:

The specified file temporary://<myFile>[_<sequenceNumber>].csv could not be moved, because the destination is invalid. More information is available in the system log.

• Error moving file

<myFile> ::= any valid CSV  file (examples I tried were correctly processed before).

<sequenceNumber> ::= a number from 0, indicating the upload atempt from the same <myFile>.csv file