I am marking this critical since it's uncaught exception at the start of the launcher. Any normal user has no chance of getting past that.
The full error&stacktrace:
Loading BuildTools version: git-BuildTools-4bf25ea-55 (#55) ************ ************@************.cz Starting clone of https://hub.spigotmc.org/stash/scm/spigot/craftbukkit.git to CraftBukkit Exception in thread "main" java.lang.IllegalArgumentException: Integer value http.postbuffer out of range at org.eclipse.jgit.lib.Config.getInt(Config.java:229) at org.eclipse.jgit.lib.Config.getInt(Config.java:208) at org.eclipse.jgit.transport.TransportHttp$HttpConfig.<init>(TransportHttp.java:209) at org.eclipse.jgit.transport.TransportHttp$3.parse(TransportHttp.java:199) at org.eclipse.jgit.transport.TransportHttp$3.parse(TransportHttp.java:197) at org.eclipse.jgit.lib.Config.get(Config.java:702) at org.eclipse.jgit.transport.TransportHttp.<init>(TransportHttp.java:244) at org.eclipse.jgit.transport.TransportHttp$1.open(TransportHttp.java:160) at org.eclipse.jgit.transport.Transport.open(Transport.java:565) at org.eclipse.jgit.transport.Transport.open(Transport.java:442) at org.eclipse.jgit.transport.Transport.open(Transport.java:323) at org.eclipse.jgit.transport.Transport.open(Transport.java:292) at org.eclipse.jgit.api.FetchCommand.call(FetchCommand.java:119) at org.eclipse.jgit.api.CloneCommand.fetch(CloneCommand.java:202) at org.eclipse.jgit.api.CloneCommand.call(CloneCommand.java:135) at org.spigotmc.builder.Builder.clone(Builder.java:636) at org.spigotmc.builder.Builder.main(Builder.java:189)
There's nothing more to it, after this error the tool terminates.