[6.3.0] - LDAP login not working for (some) existing users

Description:

When existing users log in via LDAP, some users fail to log in w/o any notification.

Steps to reproduce:

1. Configure LDAP Login
2. Login as a LDAP user and log out
3. log back in

Expected behavior:

Login should work

Actual behavior:

Login does not work. Wrong passwords show a notification, but the correct password shows no message at all in the GUI. The log shows the following entry:

{"level":50,"time":"2023-08-04T14:30:35.899Z","pid":1882221,"hostname":"rocketchat","name":"LDAP Data Converter","err":{"type":"errorClass","message":"Username already exists. [403]","stack":"Error: Username already exists. [403]<br>    at AccountsServer.insertUserDoc (packages/accounts-base/accounts_server.js:1206:15)<br>    at app/authentication/server/startup/index.js:300:28<br>    at /opt/Rocket.Chat/programs/server/npm/node_modules/meteor/promise/node_modules/meteor-promise/fiber_pool.js:43:40<br> => awaited here:<br>    at Function.Promise.await (/opt/Rocket.Chat/programs/server/npm/node_modules/meteor/promise/node_modules/meteor-promise/promise_server.js:56:12)<br>    at AccountsServer.Accounts.insertUserDoc (app/authentication/server/startup/index.js:355:17)<br>    at app/importer/server/classes/ImportDataConverter.ts:372:19<br>    at /opt/Rocket.Chat/programs/server/npm/node_modules/meteor/promise/node_modules/meteor-promise/fiber_pool.js:43:40<br> => awaited here:<br>    at Function.Promise.await (/opt/Rocket.Chat/programs/server/npm/node_modules/meteor/promise/node_modules/meteor-promise/promise_server.js:56:12)<br>    at app/importer/server/classes/ImportDataConverter.ts:523:18<br>    at /opt/Rocket.Chat/programs/server/npm/node_modules/meteor/promise/node_modules/meteor-promise/fiber_pool.js:43:40","isClientSafe":true,"error":403,"reason":"Username already exists.","errorType":"Meteor.Error"},"msg":"Username already exists. [403]"} 

Interestingly, the setting to merge existing users is already set to on.

Server Setup Information:

• Version of Rocket.Chat Server: 6.3.0
• Operating System: Debian 11
• Deployment Method: tar
• Number of Running Instances: 1
• DB Replicaset Oplog: 1
• NodeJS Version: v14.21.3
• MongoDB Version: 5.0.19

Client Setup Information

• Desktop App or Browser Version: Browser/Firefox
• Operating System: Any

Additional context

Relevant logs:

Start by reproducing the LDAP login flow and read app/authentication/server/startup/index.js, packages/accounts-base/accounts_server.js, and app/importer/server/classes/ImportDataConverter.ts around the logged error. Trace how the existing-user merge setting is applied; done means existing LDAP users can log in successfully while incorrect passwords still show the expected notification.