You’re having ConcurrentModificationExceptions that occur randomly and can not be reproduced at will. The stacktrace doesn’t help much since it simply points into VerticalLayout.add() or similar, and everything looks okay. This kind of exception happens when a Vaadin code is accessed from a background thread. But it’s definitely not easy to figure out, exactly who is calling Vaadin code without ui.access().

Session Replication

The exception may be thrown when you have session replication turned on. Unfortunately, that’s bit of a problem.

The servlet spec lacks the precise specification of at what time Tomcat (or any servlet container) is allowed to perform the session replication. Therefore, Tomcat is free to start the session replication process anytime, even if there’s an active request ongoing. That means that the following situation is entirely possible: there could be one thread manipulating UI components (the Vaadin UI thread), and another thread concurrently serializing/reading the state of those components. And that may lead to ConcurrentModificationException thrown randomly.

See Issue #7328 for more details and for a possible workaround; the workaround was found not to be perfect, for reasons that haven’t been investigated yet.

Enabling Exception Logging

Vaadin calls VaadinSession.checkHasLock() periodically, to check whether the UI code is accessed from a Vaadin UI thread (a thread which holds the Vaadin Session lock). By default, checkHasLock() only calls assert which in production does nothing, since you would have to enable asserts via -ea in your JVM, but no-one does that in production.

Starting with Vaadin 24.4, you can reconfigure checkHasLock() to do something different: either log a warning message including the stacktrace if the lock is not held, or fail with an exception. The ‘log’ variant will be immensely useful since it will contain the full stacktrace; from that you will be able to spot exactly which thread calls Vaadin UI code without obtaining the lock first. See Vaadin Flow #15776 for more details.