.zec files are the persistent zeo cache files, when you enable the cache with zeo-client-client option. IIRC there's another parameter where you can say how many object updates the zeoserver object counter and the last zeo-client-cache stored object can differ before a local zeo-client persistent cache is thrown away completely.
Maybe there is a binary difference between Python 3.7.5 and 3.7.9 (unlikely), or somehow your local zeo-client cache is still used when it should be thrown away because the zeoserver database has been replaced completely.