I have several XP VM's and its also physically one on a on a multiboot laptop. Just today, they all gave "unknown username or bad password" to the main samba server which has been working fine for years. There is nothing wrong with the accounts because Win 7 clients using the same credentials are fine. On the LAN I have Ubuntu servers and their samba shares are OK from XP. It seems its just the specific combination of XP client to Fedora samba which has this problem. I even downgraded samba to 4.5.0 but it still shows this issue.
The ubuntu version which is OK is 4.3.11
Also note when I browse the LAN from the broken XP clients in Explorer the server name comes up, so XP does know it is there (when I click the server name I'm asked to login, and can't as described. Normally these are permanently mapped to drive letters so I never even see the prompt).
Could something else have changed which causes this, like some kind of password encryption which XP can't handle, etc?