I ran Dropbox fine over several month. This week I installed Allway Sync. Now Dropbox takes over 100% of the hard drive ressources according to the task manager (not file space). The system is completely blocked. Deleting the dropbox process frees the ressources immediately and the PC operates as before. Of course, the data in the dropbox folders are not synchronized any longer with the cloud.
I am not 100% sure if this relates to Allway Sync, but that's the only thing I changed.
Updating and therefore reinstalling dropbox did not solve the problem.