Server long clickEvent problem

Please provide the exact command you used to create the sign.

uploaded

Can confirm
When restarting the server, it prints the following in the server consol and makes the sign empty:

[03:20:47 ERROR]: Failed to load data for block entity Sign
com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 1
	at com.google.gson.internal.Streams.parse(Streams.java:56) ~[minecraft_server.16w06a.jar:?]
	at com.google.gson.TreeTypeAdapter.read(TreeTypeAdapter.java:54) ~[minecraft_server.16w06a.jar:?]
	at od.a(SourceFile:501) ~[minecraft_server.16w06a.jar:?]
	at od.a(SourceFile:512) ~[minecraft_server.16w06a.jar:?]
	at eu$a.a(SourceFile:205) ~[minecraft_server.16w06a.jar:?]
	at aql.a(SourceFile:105) ~[minecraft_server.16w06a.jar:?]
	at apt.b(SourceFile:108) [minecraft_server.16w06a.jar:?]
	at asq.a(SourceFile:306) [minecraft_server.16w06a.jar:?]
	at asq.a(SourceFile:83) [minecraft_server.16w06a.jar:?]
	at asq.a(SourceFile:70) [minecraft_server.16w06a.jar:?]
	at lo.f(SourceFile:124) [minecraft_server.16w06a.jar:?]
	at lo.c(SourceFile:76) [minecraft_server.16w06a.jar:?]
	at lo.d(SourceFile:91) [minecraft_server.16w06a.jar:?]
	at net.minecraft.server.MinecraftServer.l(SourceFile:324) [minecraft_server.16w06a.jar:?]
	at net.minecraft.server.MinecraftServer.a(SourceFile:295) [minecraft_server.16w06a.jar:?]
	at la.j(SourceFile:226) [minecraft_server.16w06a.jar:?]
	at net.minecraft.server.MinecraftServer.run(SourceFile:426) [minecraft_server.16w06a.jar:?]
	at java.lang.Thread.run(Unknown Source) [?:1.8.0_51]
Caused by: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 1
	at com.google.gson.stream.JsonReader.syntaxError(JsonReader.java:1505) ~[minecraft_server.16w06a.jar:?]
	at com.google.gson.stream.JsonReader.checkLenient(JsonReader.java:1386) ~[minecraft_server.16w06a.jar:?]
	at com.google.gson.stream.JsonReader.doPeek(JsonReader.java:572) ~[minecraft_server.16w06a.jar:?]
	at com.google.gson.stream.JsonReader.peek(JsonReader.java:414) ~[minecraft_server.16w06a.jar:?]
	at com.google.gson.internal.Streams.parse(Streams.java:42) ~[minecraft_server.16w06a.jar:?]
	... 17 more

Is this still an issue in the most recent versions (currently that is 1.10.2, or 16w42a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.

In 16w42a it creates the sign, and only prints "[Server thread/WARN]: Skipping Entity with id minecraft:fallingsand" to the server console when clicked. So let's call it fixed.
Edit: After restarting the server, the sign is invisible and only shows its hitbox. You have to restart the server though, simply reconnecting does nothing.

The other things you mentioned are different issues (1 with a command (fallingsand->falling_block) the other is missing block entity data).