I encounter a problem similar to this thread:
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.