Something very strange happened suddenly, I don't know why, but here how I resolved the issue

[tbex78]

Today something very strange happened, I didn't change anything and time machine was doing its backup normaly every hour and suddenly time machine refused to backup with the time capsule. When I looked at the log there was these lines :
isKnownServer 1
Unable to find matching items -25300 (x8-10)
OpenSession failed 80

The first thing I looked was for a disk corruption, but no, absolutly nothing, the disk was fine.

Then, I added all my Bob's AirPort Time Capsule._smb._tcp.local. / Bob%27s%20AirPort%20Time%20Capsule._smb._tcp.local. / bobs-airport-time-capsule.local listed in the log to /private/var/root/Library/Group Containers/group.com.apple.NetworkAuthorization.ServerMarkers/serverMarkers.plist

with this command for example :
sudo /usr/libexec/PlistBuddy -c 'Add ":Bob%27s%20AirPort%20Time%20Capsule._smb._tcp.local." bool true' '/private/var/root/Library/Group Containers/group.com.apple.NetworkAuthorization.ServerMarkers/serverMarkers.plist'

But it still wasn't working.

then when searching on google, found this https://community.qnap.com/t/time-machine-backup-fails-with-authentication-error-80-on-tbs-h574tx/5613/9

The script mentioned in part B fix worked for me.

read -r -s "TMPW?SMB password: " && echo
for S in "Bob's AirPort Time Capsule._smb._tcp.local." "Bob%27s%20AirPort%20Time%20Capsule._smb._tcp.local." "bobs-airport-time-capsule.local"; do
  sudo security delete-internet-password -a admin -s "$S" /Library/Keychains/System.keychain 2>/dev/null
  sudo security add-internet-password \
    -a admin -s "$S" -p /Data -r 'smb ' \
    -D 'Time Machine Network Password' \
    -T /System/Library/CoreServices/NetAuthAgent.app \
    -T /System/Library/CoreServices/NetAuthAgent.app/Contents/MacOS/NetAuthSysAgent \
    -T /System/Library/PrivateFrameworks/SystemAdministration.framework/XPCServices/writeconfig.xpc \
    -T /System/Library/CoreServices/TimeMachine/backupd \
    -T /System/Library/CoreServices/TimeMachine/backupd-helper \
    -w "$TMPW" \
    /Library/Keychains/System.keychain
done
unset TMPW

Note that a . I didn't pay attention before was present at the end of some url that I didn’t used until now.

And now all work fine like before... But why all that happened so sudenly? No idea...

[jamesyc]

Looks like this is a macOS 26.4 Time Machine config issue. I will add this to FAQ

[jamesyc]

Added PR #143