after rebuilding Text index query gives no rows

Hello,

when I rebuild an Oracle Text Index, a query is no longer working.

Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production

NGEN@KURT2@SQL > select rownum, rowid, cid, ced, text from dtext partition (DTEXT_2013) where contains (text, 'sport') > 0 and rownum < 20;

    ROWNUM ROWID              CID                                                              CED       TEXT

---------- ------------------ ---------------------------------------------------------------- --------- --------------------------------------------------------------------------------

         1 AAAIj9AAFAAAr8NAAF 46-53048318                                                      03-JAN-13 <ht>Ein Mann sieht rot</ht><uz>Hirscher macht Jagd auf das rote Trikot des Weltc

         2 AAAIj9AAFAAAr8QAAD 46-55940418                                                      10-JUN-13 <gt><Index><IndexName>PRIME MARKET</IndexName><Kopf><Vortag>10.06.2013</Vortag><

         3 AAAIj9AAFAAAr8QAAL 46-54655959                                                      03-APR-13 <uez>wiener sport </uez>

         4 AAAIj9AAFAAAr8RAAA 46-53015165                                                      01-JAN-13 <uez>HOROSKOP</uez><uz>Ihre<?ZP?>Sterne<?ZP?>heute</uz><autor>REGINA<?ZP?>BINDER