Client is not registered with the server error

Classic Servoy
1

Today I see a number of entries in our server log like this:

com.servoy.j2db.persistence.RepositoryException: Client is not registered with the server. 
    	at com.servoy.j2db.dataprocessing.Zv.Za(Zv.java:304) 
    	at com.servoy.j2db.dataprocessing.Zv.performQuery(Zv.java:28) 
    	at com.servoy.j2db.dataprocessing.RowManager.rollbackFromDB(RowManager.java:279) 
    	at com.servoy.j2db.dataprocessing.RowManager.changeByOther(RowManager.java:326) 
    	at com.servoy.j2db.dataprocessing.FoundSetManager.notifyDataChange(FoundSetManager.java:1686) 
    	at com.servoy.j2db.ClientStub$5$1.run(ClientStub.java:158) 
    	at com.servoy.j2db.server.headlessclient.WebClient.invokeLater(WebClient.java:531) 
    	at com.servoy.j2db.ClientStub$5.run(ClientStub.java:143) 
    	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) 
    	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) 
    	at java.lang.Thread.run(Unknown Source)

I have not seen this before and cannot find similar in the forum. Does anyone know what causes this?

This is in Servoy 5.2.17