I can open others, and I can open it when copied to Desktop, but whenever I try to open in Dropbox I get the error message "file is in use". It has opened, once or twice, but not since upgrading to v2019.
"File is in use" is indicating that a local or remote process has attached itself to the data file, such as anti-virus scanners.
We do not recommend directly opening data files that are stored on a DropBox or other online file storage, even if that remote location is mapped to a local drive letter. Doing so increases the risk of data corruption.
The use of Dropbox for storing/sharing backup files is fine.