Bitrix24 REST API is a set of tools you can use to build apps and integrations. With REST, you can automate workflows and extend your Bitrix24 using API requests.

If you send too many REST requests or configure them incorrectly, you can overload the server. For example, if an app checks all CRM activities every five minutes, it might get blocked. Alaio Inc. can limit access to the REST API due to a high load, according to clause 18.2. (e) of the Terms of Service.
Bitrix24 Terms of Service

What happens when the load is too high

• Non-critical load. The first administrator receives a message asking them to contact support to identify the cause. If the admin does not respond, the source of the load (webhook, local app, or other REST integration) is blocked.

• High load. The source of the load is blocked immediately. The first administrator is notified. Contact support to find out why it was blocked and how to fix it.

Notifications about high load

Depending on the situation, the first administrator may receive one of three messages in chat. All account administrators also receive a notification with a link to the blocked app. If multiple integrations are blocked, the notification includes a link to only one of them.

• Your test.bitrix24.com is showing a dramatic increase in activity. This creates a heavy system and server load and affects other users.
  Did you start any resource-intensive processes such as export, import, or REST automation? Please reply to this message.

• Hello!
  Your webhook #WEBHOOK# has been blocked due to high server load. Contact support for more details.

• Hello!
  Your app #APP# has been blocked due to high server load. Contact support for more details.

You can also review REST request load in Bitrix24:
Evaluate the REST load in Bitrix24