IllegalThreadStateException when starting an Apache Camel Route with resequencer

Question

I have an Spring Boot application which uses Apache Camel for routing along with ActiveMQ as a message broker. I also use Hawtio for routing monitorization and to stop/start the routes.

Everything is working great except for one single route, the one that has a resequencer used to sort a batch of messages before processing them. I can stop this route from Hawtio, but when I try to start the route again, Hawtio gives me this error:

{
   "request": {
      "mbean": "org.apache.camel:context=camel,name=\"MAIL_READER\",type=routes",
      "attribute": "LastError",
      "type": "read"
   },
   "value": {
      "exception": {
         "routeId": "MAIL_READER",
         "localizedMessage": "Failed to start route MAIL_READER because of Route(MAIL_READER)[From[activemq:queue:MAIL_READER...",
         "cause": {
            "localizedMessage": "java.lang.IllegalThreadStateException",
            "cause": {
               "localizedMessage": null,
               "cause": null,
               "suppressed": [],
               "message": null
            },
            "suppressed": [],
            "message": "java.lang.IllegalThreadStateException"
         },
         "suppressed": [],
         "message": "Failed to start route MAIL_READER because of Route(MAIL_READER)[From[activemq:queue:MAIL_READER..."
      },
      "phase": "START"
   },
   "timestamp": 1608036365,
   "status": 200
}

If I remove the resequencer from the route, the stop/start works fine. I can pause/resume the route with the resequencer though, without any kind of problem. This is the Camel route definition in the Spring Boot application:

from("activemq:queue:mail_reader?transacted=true")
    .resequence(simple("${header.priority}")).batch().timeout(30000)
    .process(mailProcessor)
        .to("activemq:queue:processed_mails");

Seems like the resequencer is trying to use the original threads instead creating new ones when I send the route start order. Is there some kind of known problem with the Camel resequencer and threads? I found this, and it seems to be solved...

Thanks in advance.