Hi,
You can try setting 
https://www.knot-dns.cz/docs/1.6/singlehtml/index.html#ixfr-fslimit or
simply deleting the journal file corresponding to the ajetaci.cz zone.
Dan
   Good morning,
   where is problem with zone ?
   Error :
 journal size is too small to fit the changes.
   Knot is owner of this directory.
   After loading zone I found that record on some line in the zone file is
 invalid, I deleted it, increased serial and after reloading zone I could
 not set it UP
   Log records about the zone:
 Nov  9 08:22:14 celer knot[20407]: Semantic checks completed for
 zone=ajetaci.cz.
 Nov  9 08:22:14 celer knot[20407]: Zone 'ajetaci.cz.' loaded (serial
 2015110901)
 Nov  9 08:22:14 celer knot[20407]: DNSSEC: Zone ajetaci.cz. - Signing
 started...
 Nov  9 08:22:14 celer knot[20407]: DNSSEC: Zone ajetaci.cz. - - Key is
 valid, tag 36256, file Kajetaci.cz.+005+36256.private, KSK, active, public
 Nov  9 08:22:14 celer knot[20407]: DNSSEC: Zone ajetaci.cz. - - Key is
 valid, tag 11937, file Kajetaci.cz.+005+11937.private, ZSK, active, public
 Nov  9 08:22:14 celer knot[20407]: DNSSEC: Zone ajetaci.cz. - - Key is
 valid, tag 5300, file Kajetaci.cz.+007+05300.private, ZSK, active, public
 Nov  9 08:22:14 celer knot[20407]: DNSSEC: Zone ajetaci.cz. - - Key is
 valid, tag 58830, file Kajetaci.cz.+007+58830.private, KSK, active, public
 Nov  9 08:22:14 celer knot[20407]: [error] Zone 'ajetaci.cz.' journal size
 is too small to fit the changes.
 Nov  9 08:22:14 celer knot[20407]: Loaded 2 out of 3 zones.
 Nov  9 08:22:14 celer knot[20407]: [warning] Not all the zones were loaded.
   Any ideas ?
   Thanks and best regards
   J.