I have a code that used to work perfectly but, after having reinstalled R, it provides an error. What I'm trying to do is the following:
world <- ne_countries(scale = 110)
atl_extent <- extent(-69, 20, -55, 65)
study_area <- crop(world, atl_extent)
Usually, this would crop perfectly by the coordinates I provided, like this plot. 
I've tried also st_crop(world, xmin, ymin, xmax, ymax) but it produces the following error:
Error in wk_handle.wk_wkb(wkb, s2_geography_writer(oriented = oriented, :
Loop 0 is not valid: Edge 0 crosses edge 78
In the current version of
rnaturalearth,ne_countries()returns ansfobject, soraster(?) methods do not really know what to do with it.Your 2nd issue is related to less-than-ideal shapes from Natural Earth combined with
sfdefault mode when dealing with geographical coordinates -- s2 (spherical geometries), it happens to be quite picky when it comes to non-valid geometries. A simple workaround is to disable s2 for that operation.