<p>The bottom of my prints warp/curve upwards, most often at the corners. This is a very slight curve, only about 1-2 mm.</p>
<ul>
<li>I print with a raft all the time.</li>
<li>I don't have a heated bed.</li>
<li>I print with PLA at 199 °C (390 °F) with a print speed of approximately 40 mm/s</li>
<li>I have a Sindoh 3DWOX DP201.</li>
</ul>
<p>What slicer settings might be the cause of this phenomena?
Or could it be 3D printer settings?</p>

If you have a heat bed, heat it up accordingly (for example for PLA 50 °C first layers, 40 °C then can be a good starting point).
If the first layer isn't close enough, then warping can happen (Happened to me when moving from 0.3 mm layers to 0.1 mm).
If you are under extruding / have dirt in the system (the heat chamber) so that extrusion is uneven it can make warps.
And as always, you can try to print slower, it helps not always but often.
Please also do tell what temperatures (heat bed & nozzle) you are using and what material.