In snapshot 17w50a I have a structure block looping to create an unbreakable structure. It has worked just fine until the new update where activating immediately crashes the game. Has happened in every 1.13 snapshot.
---- Minecraft Crash Report ----
// I feel sad now

Time: 12/19/17 7:29 PM
Description: Exception while updating neighbours

g: Non [a-z0-9/._-] character in path of location: minecraft:Black Mesa West
at mx.<init>(SourceFile:36)
at mx.<init>(SourceFile:41)
at bai.c(SourceFile:395)
at ayj.a(SourceFile:107)
at ayj.a(SourceFile:95)
at bbi$a.a(SourceFile:419)
at aqy.a(SourceFile:486)
at auf.f(SourceFile:204)
at auf.b(SourceFile:187)
at bbi$a.a(SourceFile:434)
at bck.a(SourceFile:492)
at aqy.a(SourceFile:337)
at auf.b(SourceFile:64)
at bbi$a.a(SourceFile:444)
at qt.a(SourceFile:632)
at qt.d(SourceFile:219)
at net.minecraft.server.MinecraftServer.w(SourceFile:659)
at net.minecraft.server.MinecraftServer.v(SourceFile:592)
at clm.v(SourceFile:152)
at net.minecraft.server.MinecraftServer.run(SourceFile:497)
at java.lang.Thread.run(Thread.java:745)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– Head –
Thread: Server thread
Stacktrace:
at mx.<init>(SourceFile:36)
at mx.<init>(SourceFile:41)
at bai.c(SourceFile:395)
at ayj.a(SourceFile:107)
at ayj.a(SourceFile:95)
at bbi$a.a(SourceFile:419)

– Block being updated –
Details:
Source block type: ID #minecraft:repeater (block.minecraft.repeater // axh)
Block: minecraft:structure_block[mode=load]
Block location: World: (283,57,425), Chunk: (at 11,3,9 in 17,26; contains blocks 272,0,416 to 287,255,431), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Stacktrace:
at aqy.a(SourceFile:486)
at auf.f(SourceFile:204)
at auf.b(SourceFile:187)
at bbi$a.a(SourceFile:434)
at bck.a(SourceFile:492)
at aqy.a(SourceFile:337)
at auf.b(SourceFile:64)
at bbi$a.a(SourceFile:444)

– Block being ticked –
Details:
Block: minecraft:repeater[delay=1,facing=south,locked=false,powered=false]
Block location: World: (283,57,426), Chunk: (at 11,3,10 in 17,26; contains blocks 272,0,416 to 287,255,431), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Stacktrace:
at qt.a(SourceFile:632)
at qt.d(SourceFile:219)

– Affected level –
Details:
Level name: world
All players: 1 total; [qv['Epsi1299'/482, l='world', x=296.88, y=53.00, z=451.93]]
Chunk stats: ServerChunkCache: 4225 Drop: 0
Level seed: -2978931683083982633
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: World: (228,60,623), Chunk: (at 4,3,15 in 14,38; contains blocks 224,0,608 to 239,255,623), Region: (0,1; contains chunks 0,32 to 31,63, blocks 0,0,512 to 511,255,1023)
Level time: 14254715 game time, 24347 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 1 (now: false), thunder time: 1 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
at net.minecraft.server.MinecraftServer.w(SourceFile:659)
at net.minecraft.server.MinecraftServer.v(SourceFile:592)
at clm.v(SourceFile:152)
at net.minecraft.server.MinecraftServer.run(SourceFile:497)
at java.lang.Thread.run(Thread.java:745)

– System Details –
Details:
Minecraft Version: 17w50a
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 260005216 bytes (247 MB) / 922746880 bytes (880 MB) up to 1073741824 bytes (1024 MB)
JVM Flags: 8 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M
IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [qv['Epsi1299'/482, l='world', x=296.88, y=53.00, z=451.93]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.