Moderator Note
For some people, reinstalling fixes this issue. To do so, press your Windows+R keys, type in %AppData% at the dialog, and press Enter. In the window that appears, open the .minecraft directory, and delete everything except for the saves directory that you see therein. Then, try running Minecraft again.
[19:48:42] [Client thread/INFO]: Setting user: <username>
Exception in thread "Client thread" com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 1 column 1
at com.google.gson.Gson.fromJson(Gson.java:815)
at com.google.gson.Gson.fromJson(Gson.java:768)
at sa.b(SourceFile:148)
at sa.<init>(SourceFile:42)
at net.minecraft.server.MinecraftServer.<init>(SourceFile:154)
at cyk.<init>(SourceFile:40)
at bss.<init>(SourceFile:285)
at net.minecraft.client.main.Main.main(SourceFile:124)
Caused by: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 1 column 1
at com.google.gson.stream.JsonReader.beginArray(JsonReader.java:338)
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:79)
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:60)
at com.google.gson.Gson.fromJson(Gson.java:803)
... 7 more
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Sorry Mustek, but it looks like this is an error in a JSON file somewhere, therefore an issue with the game (also note Exception in thread "Client thread"). Considering it gives an error right at startup, it's probably not a resource pack issue.

No, it's an actual issue which many people suffer from.
Reopened and redesigned.
will it delete my servers? it did delete them 😞
Having the same issues. I have tried uninstalling and reinstalling. Still receiving the same errors.
Exception in thread "Client thread" com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 1 column 1
at com.google.gson.Gson.fromJson(Gson.java:815)
at com.google.gson.Gson.fromJson(Gson.java:768)
at lu.b(SourceFile:151)
at lu.<init>(SourceFile:43)
at net.minecraft.server.MinecraftServer.<init>(SourceFile:155)
at bpp.<init>(SourceFile:41)
at avf.<init>(SourceFile:293)
at net.minecraft.client.main.Main.main(SourceFile:124)
Caused by: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 1 column 1
at com.google.gson.stream.JsonReader.beginArray(JsonReader.java:338)
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:79)
at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:60)
at com.google.gson.Gson.fromJson(Gson.java:803)
... 7 more
Java HotSpot(TM) 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release
thank you! This worked!
For some people, reinstalling fixes this issue. To do so, press your Windows+R keys, type in %AppData% at the dialog, and press Enter. In the window that appears, open the .minecraft directory, and delete everything except for the saves directory that you see therein. Then, try running Minecraft again
Randomly started getting this error this morning (nothing changed other than what the launcher downloads).
Deleting everything but my saves/screenshots and re-installing did resolve it but felt a bit heavy-handed. Kind of curious how some people are randomly getting runtime syntax errors.
I uninstalled and reinstalled the game. That still didnt work is that the same as what is mentioned above? I dont know anything about this game as my kids are the ones who play it. Im just the one who paid for it.

Does deleting the usercache.json file in the server directory solve the problem?
Deleted the files per the moderator's recommendation- Minecraft is still not running
Rachel, did you also delete the usercache.json file that Mojang recommended?
Where can that be found? I deleted all of them except for save files in the %AppData% .minecraft section.
Go into the .minecraft folder and scroll all the way down. Most likely, it'll be under textures_4.png.
If you're still stuck, this should help: http://minecraft.gamepedia.com/.minecraft
Thank you. For mine, I am sorry that it was a duplicate issue report. But following this advice, it worked
Reoccurred in 1.8.6, here's a screenshot
This issue is occurring on my MC 1.9 server.
I have tried deleting the usercache.json
Still does not work.
I fixed this issue on my 1.10 server by deleting all of the .json files. However, when I try to join, I get the same error as if the server was not up (java.net.ConnectException: Connection refused: no further information: ). However, the server seems to be running fine...
EDIT: When I checked server.properties, all the settings were reset/messed up. I can join fine now.
Is this still an issue in the latest snapshot 16w44a? If so please update the affected versions.
This is an automated comment on any open or reopened issue with out-of-date affected versions.

Is this still a issue in the latest version of the game(currently 1.13.1)?
If so, please add it to the affected versions, thanks!
This is a technical support issue; this site is for bug reports only. For technical support please use the Mojang Support Center.