The books of Ezekiel and Jeremiah foretold that Nebuchadnezzar would attack and desolate Egypt. The Egyptians would be taken into exile, and then they would return after 40 years.
What, if any, is the evidence for this 40-year desolation of Egypt?
None, but plenty of evidence for why they might have prophesied that. I have several previous answers on the topic:
Relating to Ezekiel's prophecy of Egypt being Nebuchadnezzar's consolation for failing Tyre
Nebuchadnezzar's long history of conflict with Egypt
Specifically addressing Ezekiel's prophecies against Egypt
Evidence for Nebuchadnezzar's final attempt near the end of Ezekiel's life