Not able to invite users to any channel

Description:

I haven't found a pattern yet, but if you just type a part of a name, only "loading" will show.
On very view occations you will find the user if you type the entire user name.

E.G:
We have users named "test.user" and "test.redakteur".
If I type "test", just "loading" shows.
If i type "test.user", the user will shoiw up and is able to be invited.
If i type "test.redakteuer", just "loading" shows

I also noticed, that we can't type in special chars to the search field like space, ä, ö, ü etc

Steps to reproduce:

1. Create a public or private Channel
2. Go to member list create "invite users"
3. Start typing a name and it will show "loading" inifinite time

Expected behavior:

Users should be shown as search results

Actual behavior:

Nothing happens beyond "Loading"

Server Setup Information:

• Version of Rocket.Chat Server: 3.5.3
• Operating System: Linux
• Deployment Method:
• Number of Running Instances: 1
• DB Replicaset Oplog:
• NodeJS Version: v12.14.0
• MongoDB Version: 4.0.19

Client Setup Information

• Desktop App or Browser Version: Chrome 85.0.4183.83
• Operating System: Ubuntu 18.04

Additional context

Relevant logs:

No source file or test is named. Reproduce the issue through a public or private channel’s member list and invite-users flow using partial names and special characters, then trace the loading state and search behavior. Done means matching users appear reliably and can be invited.