It's a 'database is locked' issue.
trac-admin /var/trac repository sync INFORMATE
2018-03-12 14:13:28,519 Trac[cache] INFO: Trying to sync revision [26939]
...
trac-admin /var/trac repository sync DEMA
2018-03-12 14:13:31,882 Trac[cache] INFO: repos rev [2584] != cached rev [None]
...
the DEMA sync starts processing some big revisions
…
2018-03-12 14:13:34,203 Trac[cache] DEBUG: Caching node change in [5]: (u'tags/SP13/zMTSUsr.cls', 'file', 'add', None, -1)
2018-03-12 14:13:34,283 Trac[cache] INFO: Trying to sync revision [6]
2018-03-12 14:13:34,287 Trac[cache] DEBUG: Caching node change in [6]: (u'tags/SP14', 'dir', 'move', u'tags/SP13', 5)
2018-03-12 14:13:34,296 Trac[cache] INFO: Trying to sync revision [7]
2018-03-12 14:13:34,300 Trac[cache] DEBUG: Caching node change in [7]: (u'tags/SP14Plus', 'dir', 'add', None, -1)
2018-03-12 14:13:34,311 Trac[cache] INFO: Trying to sync revision [8]
2018-03-12 14:13:38,643 Trac[cache] WARNING: Revision 26939 already cached: OperationalError('database is locked',)
2018-03-12 14:13:39,292 Trac[cache] INFO: Resetting "repository_dir": svn:6bbf3ac7-fb38-469b-93a1-a286480b79be:/var/svn/products/INFORMATE
2018-03-12 14:13:39,310 Trac[cache] DEBUG: cache metadata undefined (youngest_rev=0)
2018-03-12 14:13:39,310 Trac[cache] INFO: repos rev [36830] != cached rev [None]
2018-03-12 14:13:39,312 Trac[cache] INFO: Trying to sync revision [1]
2018-03-12 14:13:40,258 Trac[cache] DEBUG: Caching node change in [8]: (u'tags/SP14Plus/1', 'file', 'add', None, -1)
I can work around it by stopping the background tasks while I do the initial resyncs.