Root Cause: Synapse 1.141+ explicitly blocks appservice users from using /sync. The mautrix bridges were configured to poll /sync for encryption data, which caused HTTP 500 errors.
Solution: Enabled appservice mode for end-to-bridge encryption, which uses MSC3202/MSC2409 to receive encryption data via appservice transactions instead of /sync.

In specific this means:

Changes Made

1. Synapse:
   • Added experimental features to homeserver.yaml:

           experimental_features:
                 msc3202_transaction_extensions: true
                 msc2409_to_device_messages_enabled: true

• Added org.matrix.msc3202: true to both bridge registration files

2. Signal Bridge:
   • Updated start.sh to set encryption.appservice = true
   • Built and deployed andreasdueren/mautrix-signal:1.0.2
3. WhatsApp Bridge:
   • Updated start.sh to set encryption.appservice = true
   • Built and deployed andreasdueren/mautrix-whatsapp:2.0.2

https://forum.cloudron.io/topic/14231/ccai-cloudron-custom-app-installer

I’m trying to set it up alongside the Synapse app from the Cloudron store, but I’m currently stuck with the following issue.

What I did

• Installed the Matrix (Synapse) app from the Cloudron app store
• Installed the mautrix-whatsapp bridge
• Added the registration file generated by the bridge to the Synapse app
• Restarted Synapse
• Sent login to @whatsappbot:domain.com

What happens

I consistently get:

️ Your message was not bridged: the bridge hasn't received the decryption keys

Bridge logs

The bridge fails to sync and retries indefinitely:

Dec 25 19:22:40 2025-12-25T18:22:40.932Z DBG Request completed as_user_id=@whatsappbot:domain.com duration=6.873778 method=GET req_id=477 response_length=55 response_mime=application/json status_code=500 url=https://matrix.domain.com/_matrix/client/v3/sync?filter=0&timeout=0&user_id=%40whatsappbot%3Adomain.com
Dec 25 19:22:50 2025-12-25T18:22:50.939Z ERR Error /syncing, waiting 10 seconds error="M_UNKNOWN (HTTP 500): Internal server error" component=crypto

Synapse logs (version 1.144.0)

Synapse returns HTTP 500 on /sync for the bridge user, with a NotImplementedError:

Dec 25 19:28:11 2025-12-25 18:28:11,245 - synapse.access.http.8008 - 521 - INFO - GET-887 - 172.18.20.179 - 8008 - {@whatsappbot:domain.com} Processed request: 0.004sec/0.000sec (0.001sec, 0.000sec) (0.000sec/0.001sec/1) 55B 500 "GET /_matrix/client/v3/sync?filter=0&timeout=0&user_id=%40whatsappbot%3Adomain.com HTTP/1.1" "mautrix-whatsapp/v25.12+dev.1f06bd38 mautrix-go/v0.26.1 go/1.25.5" [0 dbevts]

Followed by this stacktrace:

Dec 25 19:28:21 2025-12-25 18:28:21,251 - synapse.http.server - 147 - ERROR - GET-892 - Failed handle request via 'SyncRestServlet': <XForwardedForRequest at 0x7657785cd910 method='GET' uri='/_matrix/client/v3/sync?filter=0&timeout=0&user_id=%40whatsappbot%3Adomain.com' clientproto='HTTP/1.1' site='8008'>
Dec 25 19:28:21 Traceback (most recent call last):
Dec 25 19:28:21 File "/app/code/env/lib/python3.12/site-packages/synapse/http/server.py", line 335, in _async_render_wrapper
Dec 25 19:28:21 callback_return = await self._async_render(request)
Dec 25 19:28:21 File "/app/code/env/lib/python3.12/site-packages/synapse/http/server.py", line 576, in _async_render
Dec 25 19:28:21 callback_return = await raw_callback_return
Dec 25 19:28:21 File "/app/code/env/lib/python3.12/site-packages/synapse/rest/client/sync.py", line 269, in on_GET
Dec 25 19:28:21 sync_result = await self.sync_handler.wait_for_sync_for_user(
Dec 25 19:28:21 File "/app/code/env/lib/python3.12/site-packages/synapse/handlers/sync.py", line 358, in wait_for_sync_for_user
Dec 25 19:28:21 res = await self.response_cache.wrap(
Dec 25 19:28:21 File "/app/code/env/lib/python3.12/site-packages/synapse/handlers/sync.py", line 527, in current_sync_for_user
Dec 25 19:28:21 sync_result = await self.generate_sync_result(
Dec 25 19:28:21 File "/app/code/env/lib/python3.12/site-packages/synapse/handlers/sync.py", line 1660, in generate_sync_result
Dec 25 19:28:21 raise NotImplementedError()
Dec 25 19:28:21 NotImplementedError

Summary

It looks like the bridge calls:

/_matrix/client/v3/sync?filter=0&timeout=0

for @whatsappbot, which causes Synapse 1.144.0 to return HTTP 500 with a NotImplementedError.
Because /sync fails, the bridge can’t receive encryption keys and messages are not bridged.

Do you have any idea what could be wrong here ?
I didn’t manage to find a solution in the official documentation.

andreasdueren/mautrix-whatsapp-cloudron:1.2.4

Is there a way to set this up on the app/data side so it is persistent through updates?

Here are the options I'm looking at:
https://github.com/mautrix/zulip
https://github.com/GearKite/MatrixZulipBridge

https://git.due.ren/andreas/mautrix-whatsapp-cloudron/releases/tag/v2.0.0

"extensions" to apps by third parties is quite a burden for app packages.

I understand that it needs to be feasible to maintain these kind of things. Cloudron needs to be profitable after all. I hope we can find some solution for these things, especially if they're simple go binaries like this one.

Maybe niche is not the right term but I simply meant to say is that "extensions" to apps by third parties is quite a burden for app packages. It's best to figure out a way by which other people can host it easily especially when people are making their efforts public.

I think we have to figure a way to get such niche app packages published.

I would argue that this is equal or even less of a niche app than the Valheim Gameserver for example.

such niche app packages

I'm not sure it's really a niche app. I'd guess that many/ most people running Matrix/ Element might like to use this (and other similar bridges too). IMHO ideally Cloudron Matrix would come bundled with WhatsApp, Signal and Telegram bridges much like Element One does.

https://git.due.ren/andreas/mautrix-whatsapp-cloudron/releases/tag/v1.2.3

https://git.due.ren/andreas/mautrix-whatsapp-cloudron/releases/tag/v1.2.2

https://git.due.ren/andreas/mautrix-whatsapp-cloudron