I have been constantly getting the very common 'n must be positive' crash, which i thought was related to the lava lake thing, but no. after a long test, i discovered the issue lies with the randomize button on the ores page. something which commonly happens is that the 'min height' gets set HIGHER than the 'max height', because they are independent values which require common sense to set correctly. unfortunately mojang didn't have the common sense to give the randomize button common sense...

here is an example:
coal ore min height: 237
coal ore max height: 124

the max is smallest and the min is biggest. gj mojang.