'Apex request count has exceeded limit' error in Flow

Question

I got the following error: Error Occurred During Flow "Send Slack Messages": Apex request count has exceeded limit.

The "Send Slack Messages" flow is a custom flow that simply loops through a collection of an Apex-defined type SlackMessage and sends them using the Send Slack Message action from Flow Core Actions for Slack.

The flow is being called from an Apex queueable, something like this:

public class SlackMessageSenderWithFlow implements SlackMessageSender {

    private static final String FLOW_SEND_SLACK_MESSAGE = 'Send_Slack_Messages';

    public void send(List<SlackMessage> messages) {
        Flow.Interview.createInterview(FLOW_SEND_SLACK_MESSAGE, new Map<String, Object> {
                'vc_SlackMessages' => messages
        }).start();
    }
}

public class AQueueable implements Queueable {

    public void execute(QueueableContext ctx) {
        // ...
        sender.send(messages);
    }
}

The error is thrown with just 2 messages, so it's not a callout limit related error.

Is this error due to some kind of limitation of Salesforce flows? The documentation about the Flow.Interview class doesn't state any limitations regarding this situation. I also have not been able to find any information about this error message.