Help: seeing "Cannot overwrite" in the log when trying to rebuild lucene index - how to fix??

[MV-dev1]

In general I would like to better understand how OrientDB works and how much I can mess with the index files...   Can I just delete them manually, reload the database and have everything work?  Which files?  ;))))   Is this documented anywhere?

My problem -- I try to rebuild some indexes and see this in the log but have no idea how to help the OrientDB to correct it.

2015-08-29 06:40:55:289 INFO  {db=TestDb} - Rebuilding index TestDb.L1.l (estimated 55,870 items)... [OIndexRebuildOutputListener]{db=TestDb} Error on adding new document 'Document<stored,indexed,omitNorms<RID:#42:35488> indexed,tokenized,omitNorms,indexOptions=DOCS_ONLY<location:> stored<location:16.100000 49.800000>>' to Lucene index
java.io.IOException: Cannot overwrite: C:\orientdb\databases\TestDb\luceneIndexes
\L1.l\_0.cfs
        at org.apache.lucene.store.FSDirectory.ensureCanWrite(FSDirectory.java:293)
        at org.apache.lucene.store.FSDirectory.createOutput(FSDirectory.java:282

I have 3 different Lucene indexes and this is now only happening on this one -- for the others I can drop and recreate.  Not sure why this one is special -- the Class is even derived from a different one that does work.

CREATE CLASS LL EXTENDS V ABSTRACT;
    CREATE PROPERTY LL.y DOUBLE;
    ALTER PROPERTY LL.y NOTNULL true;
    ALTER PROPERTY LL.y MANDATORY true;
    CREATE PROPERTY LL.x DOUBLE;
    ALTER PROPERTY LL.x NOTNULL true;
    ALTER PROPERTY LL.x MANDATORY true;

CREATE CLASS L1 EXTENDS LL;
    CREATE INDEX L1.i ON L1(y,x) UNIQUE_HASH_INDEX;
    CREATE INDEX L1.l ON L1(y,x) SPATIAL ENGINE LUCENE;

CREATE CLASS L2 EXTENDS LL;
    CREATE INDEX L2.i ON L2(y,x) UNIQUE_HASH_INDEX;
    CREATE INDEX L2.l ON L2(y,x) SPATIAL ENGINE LUCENE;

'i' is a normal index that I need to update quickly
'l' is a Lucene index that I want to query from using spatial commands

PS  And, yes, I did try deleting the entire folder and reloading.

[MV-dev1]

Here are the relevant files for l2 and l2 (arrow to the file it says that it can't overwrite)...