phpList.org

Weird behaviour in unsubscribe and forward link of new users v3.6.0

Hi, I’ve upgraded to 3.6 (from 3.5) few weeks ago and everything seemed to be fine, now I’ve discovered that when I add new users, sometimes (not always) the [forward, unsubscribe, etc.] link in their message don’t work.
I got the message:
“Not Found
The requested document was not found on this server
Please contact the [Administrator](mailto:phplist@blabla.org?subject=File not Found: /lists/lt.php?tid=cR8BBQRSAw9eARRSVwEESwIEBAFLVQJbU09YA1MFUAldUggGAVYdDlxbXVEAUQBLUgcGAUsABVpVT1gHUQceAQoGDlMFBFRYAFUKTFVXBVcCAAFRSwNdAA5PCAZXUB4BAVoLHwFaUQkKVQsEUwNQAA)”
If I cancel and reinsert the email, then it works.
I don’t see any difference in the syntax of the link before and after (it doesn’t seem to be broken).
Many thanks for any help,
barbara