Clients incorrectly denied permission to view some events

[matrixbot]

This issue has been migrated from #10369.

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Description

Loading the start of the time timeline in some rooms fails.

Steps to reproduce

• find a room that is marked unread in element-web
• try to access the first message
• You get the "Failed to load timeline position: tried to load a specific point in this room's timeline, but you do not have permission to view the message in question." error message

Note: this does not apply to all rooms, but in our instance to a lot of rooms.

Expectation: old events can always be loaded.

Further investigating shows that a request to

https://ungleich.matrix.ungleich.cloud/_matrix/client/r0/rooms/!qlfxvXwtSjmInWnicT%3Aungleich.ch/context/%24PsaDH5PnSV5u4lx3-06R2R9-vbDWUU8ErR2hOntC5e4?filter=%7B%22lazy_load_members%22%3Atrue%7D

fails with:

{"errcode":"M_FORBIDDEN","error":"You don't have permission to access that event."}

I first thought that might be related to us running matrix-org/rust-synapse-compress-state#27 , however we only compressed rooms with 10k+ events and many rooms that are much smaller are affected.

The users trying to access the events are often even an admin in the room, so a permission issue is unlikely.

Version information

• Homeserver: ungleich.matrix.ungleich.cloud

If not matrix.org:

• Version: {"server_version":"1.37.1","python_version":"3.7.3"}

• Install method: package manager

• Platform: Debian 10, no containers

[verymilan]

This appears to also happen at tchncs.de for at least the Whatsapp bridge. It also cannot decrypt events which I wonder whether it's related or not. At least the log is flooded with those permission errors.

• Debian 12
• From pip
• 1.98.0