Hello,
Windows XP after updating to FRITZ!OS 7.21 or higher, you need to re-enable the outdated SMB1 protocol. Go to Heimnetz > USB / Speicher > Speicher (NAS) in the FRITZ!Box interface
EZDriveMA and check "SMBv1 support (not recommended)." However, using SMB1 is insecure, so it's better to upgrade to a newer OS if possible.