Incompatible Render Engine

WHY DO I SEE A CYAN AND BLACK CHECKERS TEXTURE?

The short answer is that the material you are using is not compatible with the current render engine.
If you used a CYCLES only material and you have eevee as your render engine you will see the checkers texture.
Also if you use a CYCLES only material, you have CYCLES as your render engine but you use MATERIAL PREVIEW instead of RENDERED preview (Material Preview uses always EEVEE even if your selected render engine is CYCLES).
More about material preview

WHY WAS THIS IMPLEMENTED?

In order to prevent bad experiences with slow viewport and really long processing times that could even lead to blender crashing
The materials that are only compatible with one render engine won’t work correctly and will cause issues, so the best way to deal with it is not to process them.

WHICH MATERIALS ARE AFFECTED?

Only engine specific materials marked with a letter overlay icon in their thumbnails, E for eevee only C for Cycles only. Materials that have no letter are compatible with both render engines.

DOES IT CHANGE ANYTHING ABOUT AVAILABLE MATERIALS?

No, you can still use the same materials that you could use before. A material that was not compatible with the render engine you were using before just didn’t work and potentially slowed down your viewport greatly. Now you will not experience those issues and you will immediately notice that the material is just not compatible.

CAN I DO ANYTHING TO USE A NON COMPATIBLE MATERIAL?

CYCLES only materials in EEVEE = YES
EEVEE only materials in CYCLES (only a few in the library) – NO
In some cases yes. If you want to use a CYCLES only material in EEVEE, you can bake it using Sanctus Library baking tool.
EEVEE Only materials are quite rare, usually TOON materials or alternate version of Cycles materials made specially for eevee and in that case it is not possible since baking always uses CYCLES.