Connecting session to backup fails - failed to decrypt sessions

[HarHarLinks]

Steps to reproduce

1. Think you have key backup enabled
2. go to settings to check
3. see button "Connect this session to Key Backup"
4. better click it to be safe, don't want to lose my message history!
5. hit OK
6. back to 1

Outcome

What did you expect?

Backup works/gets enabled

What happened instead?

Keys restored
Successfully restored 8575 keys
Failed to decrypt 501 sessions!

It is not at all explained what this (failed part) means, nor does it lead to a failure mode of having failed to set up the backup

Operating system

arch

Browser information

Firefox 121

URL for webapp

https://develop.element.io

Application version

Element version: e6543bb-react-38f791b79db5-js-35ea144bcaed Crypto version: Olm 3.2.15

Homeserver

matrix.org

Will you send logs?

Yes

[akoyaxd]

I experience the same issue since round about 2 weeks. Can't tell exactly. Maybe noteworthy: I use Element X for Android and iOS on parallel with the legacy apps and Element web. Maybe rustsdk and legacy apps are conflicting in backups?

Also this issue seems to only affect messages send from others. I can always decrypt my own messages. I am happy to help collecting logs or providing logs from my homeserver. A hint what to look for would be helpful thought.

[HarHarLinks]

Ftr, I have all kinds of clients, incl legacy and X clients on Android and iOs in use on that account. The issue appeared before migration to Web R and is still present after.

[akoyaxd]

Another side note:
manually exporting and importing my e2ee keys from devices that still have the keys for the failing olm sessions works. But this does not seem to fix the backup. On new Devices I still get the error, that some sessions can not be imported.

[richvdh]

Duplicate of #26530