The author solved this by deleting all .rstudio/ config files. It did work, but the downside is you lose all RStudio settings.
My solution: delete .rstudio/monitored/connection-history-database.json
then you connection pane restored to the initial state and all other settings remain intact.
PS
The config folders are usually in your home directory and hidden. Use ls -l to locate them.
The config folders won't go away by simply uninstall the R/R Studio. So you have to delete them manually.
I had the same problem, but I haven't been able to solve it yet. I updated the R studio version, but I still have the problem! I uninstalled and installed again, and the problem persists. I deleted the files and nothing! Please, can anyone help me?