Since a few days, Dropbox requires its syncing folder to be on a compatible file system, which at the moment is only NTFS on Windows. If you are using DropboxPortableAHK on an internal hard drive, it is most likely already formatted as NTFS and everything works fine.
This issue is also mentioned in the Dropbox Help and raised hundreds of complaints in their own forums. Unfortunately, it looks like Dropbox will not change this behavior, so the only way to keep using DropboxPortableAHK on your thumb drive is to make sure it is formatted as NTFS. Please make a copy of all your data on the thumb drive before formatting it, because formatting will erase all data. Please check out the Microsoft documentation for more information about formatting a drive.
In case you regularly use DropboxPortableAHK and it makes your life easier, please consider a donation.