"hadabend" is short for HAD an ABnormal END. It means the session didn't shut down cleanly, and the user probably saw an error when resuming that said "Your R session crashed".
The most common cause of these errors is crashing native code in R packages. It's hard to know more without more context. If you're using RStudio Server, you might turn on crash reporting to collect some dumps for analysis:
https://docs.rstudio.com/ide/server-pro/server-management.html#automated-crash-reporting