Congratulation, you just found a bug !
It got introduced with the new LLDiriterator usage in the file selector: the new directory iterator code does not know how to iterate on an empty path (i.e. on the logical drives list) under Windows... Fixed for next release.
In the mean time and as a work-around, you may still force the viewer to change the cache directory as follow:
- Launch the viewer (no need to log in).
- Open the debug settings floater (CTRL ALT S or from the Advanced menu).
- Type "newc" in the "Filter" input line and when it displays "NewCacheLocation", enter the new path in the input line for the setting at the bottom of the floater (e.g. D:\ViewerCache).
- Close the floater and quit the viewer.
On next restart, the viewer will clear the old cache and setup a new one in your chosen location.