Maptile fetched in RStudio Desktop but not RStudio Server

I have some custom map tiles which were showing when I ran the app in RStudio Desktop but in RStudio Server I see a greyed-out area. I do see the marker though. How can I fix this please?

Using the code below:

mymap <- leaflet() %>% 
  addTiles("http:/{z}/{x}/{y}.png", option = tileOptions(tms = TRUE), leafletOptions(minZoom = 0, maxZoom = 17)) %>% 
  setView(-0.3224, 51.2322,  10) %>% 
  addMarkers(-0.3224, 51.2322)

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