Hi Patrik,
Could you please let me know which version of Kentico
CMS are you using?
Have you done any customization to
WYSIWYG Editor?
Which browser are you using? Have you tried different one?
Are you using any browser's extensions/plug-ins?
Please let me know whether you are using custom URL extensions?
Are there any logs related to this problem kept in Event log (Site manager -> Administration -> Event log)?
Best regards,
Michal Legen