Hello!

After upgrading EPiServer.Framework from 9.5.0 to 10.10.4 and EPiServer.Find from 11.0.0.3701 to 12.6.1 we can't clear the Find index.

When I'm trying to clear our index from the editor mode I get an error saying "Unable to load /episerver/Find/proxy/_clear?clear_index=true&clear_stats=false status: 400".

After looking closer in the logs from our on-premise Find installation, this is the exact error from elasticsearch:

[2017-10-27 10:01:57,294][DEBUG][action.index             ] [ElectroCute] [my_index][3], node[MZVkl9ShQ2Op2mS2QtowqQ], [P], s[STARTED]: Failed to execute [index {[my_index][_clear][xmWM6b5YReKe7zoDsYv40Q], source[_na_]}]
org.elasticsearch.indices.InvalidTypeNameException: mapping type name [_clear] can't start with '_'
	at org.elasticsearch.index.mapper.MapperService.add(MapperService.java:198)
	at org.elasticsearch.index.mapper.MapperService.add(MapperService.java:186)
	at org.elasticsearch.index.mapper.MapperService.documentMapperWithAutoCreate(MapperService.java:393)
	at org.elasticsearch.index.shard.service.InternalIndexShard.prepareCreate(InternalIndexShard.java:287)
	at org.elasticsearch.action.index.TransportIndexAction.shardOperationOnPrimary(TransportIndexAction.java:210)
	at org.elasticsearch.action.support.replication.TransportShardReplicationOperationAction$AsyncShardOperationAction.performOnPrimary(TransportShardReplicationOperationAction.java:532)
	at org.elasticsearch.action.support.replication.TransportShardReplicationOperationAction$AsyncShardOperationAction$1.run(TransportShardReplicationOperationAction.java:430)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.lang.Thread.run(Unknown Source)

Note that this does not happen when I try with a Find index hosted by EPi, this is only for our on-premise solution.