All scheduled cores encountered errors in user code (unexpected symbol) - but no such code

Question

/EDIT: My problem has been resolved. It turns out it was indeed an error unrelated to parallel.

Hi StackOverflow Community,

today I have a rather odd problem. I have a function which I am calling to run on 8 cores, and it has a parameter called type. Running it with type = "points" works perfectly, but I am concerned about running it with type = "lines". Even though it follows almost the same code, at some point the script crashes, and all what I get is (after almost 2 days of running!):

Warning message: 
In mclapply(cluster_times, the_nt_function, all_trips, ellipses,  :
  all scheduled cores encountered errors in user code
Error: unexpected symbol in:
"
endtime"
Execution halted

The first thing I checked is, of course, the code of the_nt_function. I provide a summary of the code below.

The important things here, are:

1. The code for type = "points" runs perfectly and gives the expected output
2. The code for type = "lines" seems to run perfectly for nearly 2 days until the error stated above appears
3. The error stated above is inexplicable: there is no 'endtime' or anything in the code below!

the_nt_function takes some inputs, and for each row in times_df it creates a kind of sample of dat. If type="points", it then counts either how many (geographical) points in dat are in the ell object, using st_intersection from sf. If type="lines", it basically does the same, but instead of counting points intersecting with the ell object, it counts lines intersecting with the ell object.

I let the code run on a high performance cluster (but only using 8 cores). On my computer, it runs fine on 7 cores, at least for a test set of seven entries in the times_df object. It does not crash but gives the expected output.

I believe that the error is not in the_nt_function. I've googled it and looked for similar problems here on stackoverflow, but the only thing I found so far is this (which I am now trying out of desperation).

Do you have any ideas, what this error wants to tell me?

/Edit: I call the function in parallel by:

system.time(
    out <- mclapply(cluster_times, the_nt_function, all_trips, ellipses, TRUE, opts$type, mc.cores = cpus)
  )

where I give the arguments in correct order.

the_nt_function <- function(times_df, dat, sf_object, type) {

  times_df$N_t = as.numeric(NA)
  
  # Here is code that performs some kind of preprocessing and filtering down data, basically creating
  # the data.table dat_filt_spc
    

    for every row in times_df {
      # do some more filtering and end up with a data table called dat_filt_time
      
      if(type == "points"){
      # Convert filtered DT to sf object (using startloc)
        suppressWarnings(
          dat_start_sf <- st_as_sf(
            as.data.frame(dat_filt_time),
            coords = c("startloclon", "startloclat"), 
            crs = 4326
          )
        )
      } else if(type == "lines") {
        suppressWarnings(
          dat_start_sf <- st_as_sf(
            as.data.frame(dat_filt_time),
            crs = 4326
            )
        )
      }
      
      # Intersect startlocs with ellipse
      suppressMessages(
        start_intersect <- st_intersection(dat_start_sf, ell)$tripid
      )
      
      # Convert filtered DT to sf object (using endloc) and filter out trips which are already intersected
      if(type == "points"){
        suppressWarnings(
          dat_end_sf <- st_as_sf(
            as.data.frame(dat_filt_time[!tripid %in% start_intersect]),
            coords = c("endloclon", "endloclat"), 
            crs = 4326
            )
        )
        # Intersect endlocs with ellipse
        suppressMessages(
          end_intersect <- st_intersection(dat_end_sf, ell)$tripid
        )
        # concatenate start and endloc intersections, assess Nt and add to times_df
        trips_intersect <- unique(c(start_intersect, end_intersect))
      } else if(type == "lines"){
        trips_intersect <- start_intersect
      }

      times_df[i, "N_t"] <- length(trips_intersect)
    }
    return(times_df)
  }
}