Description
A simple showcase of this error can be found here: https://github.com/bennypi/thymeleaf-and-springboot
Expected behaviour
The expected behaviour can be seen when changing the spring boot version to 2.5.6. With this version, the linked css
can be retrieved and the form POST is working.
Current behaviour
Starting with spring boot version 2.6.0 and still present in 2.6.1, requests with the jsessionid parameter are not processed correctly but lead to errors on the server. A call to a CSS-file with the jsessionid parameter leads to a 404, the form POST request with the jsessionid parameter also leads to a 404. The page starts working again when the page is refreshed in the browser because the browser then sends a cookie with in the request which in turn removes the jsessionid parameter in the links in the HTML.
I am not completely sure if this indeed a bug in spring boot or if thymeleaf must make changes so that it is compatible with spring boot 2.6.0 and newer, but I don't see anything in the release notes that could be a reason for this changed behaviour.