So I have some information on the problem and a temporary fix for maps.
A material referenced in a vmf file with a leading slash will no longer load.
For example, if the path to your material is "/mymaterial", csgo will look for the material in "materials//mymaterial" which is incorrect.
Use path "mymaterial" instead so that csgo loads from "materials/mymaterial".
(You don't need to change the actual location of your material, just the reference to the locations.)
This can be edited very quickly if you have the original vmf file, or if you have a decompiled version that actually works. Using notepad++, you can do the following replacement to instantly correct the vmf file, and use it to recompile the map:
If you do not have a functional vmf, the process is more tedious and is as follows:
1) Run BSPInfo
(part of BSPSource) to identify the incorrect references.
(Those with two slashes.)
2) Perform the following replacement for each of the materials that you identified
Find all instances of the string that begins with a null, then has a slash, then has the material name, then ends with a null. And replace them with the string that begins with a null, then has the material name, then ends with two nulls.
Most text editors can not insert null bytes and so you can accomplish this with perl like so:
perl -pe 's/\0\/STARWARSWALLS4\0/\0STARWARSWALLS4\0\0/g' surf_deathstar_tat.bsp > surf_deathstar_temp.bsp; mv surf_deathstar_temp.bsp surf_deathstar_tat.bsp
In the above example, the material I am fixing is "STARWARSWALLS4".
You can also do this replacement with sublime 3 as follows, however note that this text editor is extremely slow, and I do not recommend it: