Federation is not working - crypt.error ([InvalidAsn1Error]: Expected 0x30: got 0xe2)

Description:

Federation is not working.
I have a setup of two Rocket.Chat servers with "Federation" enabled. According to the "Test setup" button it should work fine on both sites. Also i am able to search and add an external user from the other server, but the messages don't get transmitted.
The log of Rocket.Chat shows "crypt.error ([InvalidAsn1Error]: Expected 0x30: got 0xe2)"
(for details see "Relevant logs" below).

Steps to reproduce:

1. setup Federation on two Rocket.Chat servers (standard snap installation)
2. add external user (on one site / or on both sites)
3. write a chat message to this external user

Expected behavior:

The chat message should be trasnmitted to the user on the other Rocket.Chat server.

Actual behavior:

Nothing happens - the user won't be notified about an arriving chat message. Only an error message occurres in Rocket.Chat logs.
(for details see "Relevant logs" below)

Server Setup Information:

• Version of Rocket.Chat Server: 3.15.0
• Operating System: Ubuntu 20.04
• Deployment Method: snap
• Number of Running Instances: 2
• NodeJS Version: v12.22.1
• MongoDB Version: 3.6.14 / wiredTiger (oplog activated)

Client Setup Information

• Desktop App or Browser Version: Google Chrome Version 91.0.4472 (or Rocket.Chat App)
• Operating System: Windows 10 / Manjaro Linux

Relevant logs:

I20210706-20:15:09.343(2) server.js:204 Federation ➔ crypt.error Error [InvalidAsn1Error]: Expected 0x30: got 0xe2
at newInvalidAsn1Error (/snap/rocketchat-server/1478/programs/server/npm/node_modules/asn1/lib/ber/errors.js:7:13)
at Reader.readString (/snap/rocketchat-server/1478/programs/server/npm/node_modules/asn1/lib/ber/reader.js:169:11)
at Object.publicImport (/snap/rocketchat-server/1478/programs/server/npm/node_modules/node-rsa/src/formats/pkcs8.js:154:44)
at Object.detectAndImport (/snap/rocketchat-server/1478/programs/server/npm/node_modules/node-rsa/src/formats/formats.js:66:48)
at NodeRSA.module.exports.NodeRSA.importKey (/snap/rocketchat-server/1478/programs/server/npm/node_modules/node-rsa/src/NodeRSA.js:183:22)
at new NodeRSA (/snap/rocketchat-server/1478/programs/server/npm/node_modules/node-rsa/src/NodeRSA.js:73:18)
at FederationKeysModel.loadKey (app/models/server/models/FederationKeys.js:19:10)
at encrypt (app/federation/server/lib/crypt.js:58:25)
at federationRequest (app/federation/server/lib/http.js:16:11)
at federationRequestToPeer (app/federation/server/lib/http.js:42:12)
at dispatchEvents (app/federation/server/handler/index.js:65:3)
at dispatchEvent (app/federation/server/handler/index.js:70:2)
at app/federation/server/hooks/afterSaveMessage.js:26:2
at /snap/rocketchat-server/1478/programs/server/npm/node_modules/meteor/promise/node_modules/meteor-promise/fiber_pool.js:43:40

I20210706-20:15:09.344(2) server.js:204 Federation ➔ http.error [IGNORED] Error Error: Could not encrypt

Start with app/models/server/models/FederationKeys.js at loadKey, then trace the stack through app/federation/server/lib/crypt.js and app/federation/server/lib/http.js. Reproduce a message between the two configured Federation servers and determine why key handling reaches the ASN.1 error. Done means messages are transmitted and the Federation crypt.error no longer occurs.