When I attempt to launch snapshot 18w21a (and I have also had problems launching from previous snapshots) I crash a few seconds after launching. In the 'latest.log' file it states the error:

java.lang.StackOverflowError: nulljava.lang.StackOverflowError: null

at java.util.Objects.equals(Objects.java:59) ~[?:1.8.0_25]

at abc$o.a(SourceFile:296) ~[18w21a.jar:?]

at abc$g.a(SourceFile:178) ~[18w21a.jar:?]

at abc$m.a(SourceFile:526) ~[18w21a.jar:?]

at abc$m.a(SourceFile:526) ~[18w21a.jar:?]

at aat.b(SourceFile:47) ~[18w21a.jar:?]

at abc$l.a(SourceFile:610) ~[18w21a.jar:?]

at abc$m.a(SourceFile:530) ~[18w21a.jar:?]

...and then repeats the part in bold until the end of the log (presumably when it crashes).

You can see the full log in the file attached.