I also removed all the database connections from my app, still no luck. There is no error in the log, only messages for loading packages.
I tried a simple app and I can run multiple instance at the same time, so there must be something wrong with my code... But I didn't make any changes to my code for nearly a month
I moved all the code and data files for my shiny server to another server, and it works just fine with multiple users using it at the same time
EDIT: just checked the most recent logs, and found "8066 Segmentation fault (core dumped) R --no-save --slave -f /opt/shiny-server/R/SockJSAdapter.R" at the bottom of the log file