[xwiki-users] User registration

Hi, We have migrated to xwiki 8.2.1 in a docker environment. Everything is working as a charm. However: When registerring a user (as an admin), I get an error. Investigating this only gives me results that date from 2013 but nothing recent. I am not sure how to proceed. The error is: <error> type Exception report message Servlet execution threw an exception description The server encountered an internal error that prevented it from fulfilling this request. exception javax.servlet.ServletException: Servlet execution threw an exception com.xpn.xwiki.web.ActionFilter.doFilter(ActionFilter.java:115) org.xwiki.wysiwyg.server.filter.ConversionFilter.doFilter(ConversionFilter.java:127) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) org.xwiki.container.servlet.filters.internal.SetHTTPHeaderFilter.doFilter(SetHTTPHeaderFilter.java:63) org.xwiki.container.servlet.filters.internal.SavedRequestRestorerFilter.doFilter(SavedRequestRestorerFilter.java:208) org.xwiki.container.servlet.filters.internal.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:111) org.xwiki.resource.servlet.RoutingFilter.doFilter(RoutingFilter.java:137) root cause java.lang.NoClassDefFoundError: Could not initialize class com.xpn.xwiki.web.RegisterAction sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) java.lang.reflect.Constructor.newInstance(Constructor.java:423) java.lang.Class.newInstance(Class.java:442) org.apache.struts.util.RequestUtils.applicationInstance(RequestUtils.java:172) org.apache.struts.util.RequestUtils.applicationInstance(RequestUtils.java:147) org.apache.struts.action.RequestProcessor.processActionCreate(RequestProcessor.java:281) org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:221) org.apache.struts.action.ActionServlet.process(ActionServlet.java:1913) org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:449) javax.servlet.http.HttpServlet.service(HttpServlet.java:622) javax.servlet.http.HttpServlet.service(HttpServlet.java:729) com.xpn.xwiki.web.ActionFilter.doFilter(ActionFilter.java:115) org.xwiki.wysiwyg.server.filter.ConversionFilter.doFilter(ConversionFilter.java:127) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) org.xwiki.container.servlet.filters.internal.SetHTTPHeaderFilter.doFilter(SetHTTPHeaderFilter.java:63) org.xwiki.container.servlet.filters.internal.SavedRequestRestorerFilter.doFilter(SavedRequestRestorerFilter.java:208) org.xwiki.container.servlet.filters.internal.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:111) org.xwiki.resource.servlet.RoutingFilter.doFilter(RoutingFilter.java:137) </error> Is there anyone who can show the direction to look for an answer? Br, Peter