Istvan-Zoltan,

Did you upgrade directly or did you follow the suggested upgrade path?

When there is nothing in the event log it could be a client-side (browser) problem. Did you try another browser, did it work there?

Are you using a third-party skin? If yes, does it work with a standard skin?

Next thing I would try is upgrading all used extensions (modules, skins, ...) with the upgrade of DNN. This can be done easily by copying the install packages to the corresponding folders in the /install folder (modules, skins,...) before starting the upgrade process.

If this does not help, I would start uninstalling all third party extensions before the upgrade, and then re-install them after the upgrade, check if the registration works and find out which extension is causing the issue. Then try to get an upgrade for this extension(s) before upgrading DNN.

All of this has to be done in a testing environment and not on your production site - it requires a lot of backups and restores, but it's worth the effort.

Happy DNNing!
Michael