Solution : https://service.sap.com/sap/support/notes/1594554 (SAP Service marketplace login required)
Key words : 
trex/bwa indexserver trace, 6926 unsupported attribute file version, general search engine error, [sparse_check]            set_sparse_check = 1            prepare_renumber_threads = 1, avoids parallel execution, indexing searching rollup, applying sparse types, file trexindexserver, attribute compression, bwa indexes
Related Notes : 
       
| 1594611 | BWA 7.20: Revision 12 | 
| 1393505 | BWA 7.20: Central Note |