Slack "Send Channel Message" action fails with invalid_json on every connection

In our Pabbly Connect account, the Slack action "Send Channel Message" fails on every execution with this response:

{"ok": false, "error": "invalid_json"}

What we have verified:

1. It fails with three different Slack connections: two from 2023 ("FAST BOT" and "Slack Igor") and one brand-new connection created on November 1, 2026 ("FAST BOT 2026"). All connections pass your "verify" check.

2. It fails for every message format: text mapped from a previous step, a plain constant single-line text, and the block message format.

3. The target channel is correct and verified (channel "system-preview", ID C03PKA35VU0). No message ever arrives in the channel.

4. Other apps in the same workflows work fine (ClickUp and Google Contacts actions execute successfully), so this is specific to the Slack app integration.

5. One of the old connections showed a refreshed date on your Connections page after re-authentication on November 1, 2026, but its underlying refresh timestamp never actually changed, which suggests the re-authentication is not completing.

This affects many workflows on the account (the "FAST BOT" Slack connection alone is used by 32 workflows), including a production status-monitoring workflow whose Slack reminders are not being delivered.

Please investigate the Slack app integration for this account and advise. We are happy to provide workflow links or task history IDs on request.

Thank you.