Ok: I've exported the database and installed Joplin in both machines from scratch. Then, I imported back the exported file in one of the machines. Next made the second machine to synchronize with the first. It did alright. After that I changed a file on the first machine and saved it. Alas, it did not synchronize.

Here is the file that was copied from the first to the second machine when this last one was first synchronized:

rw-rw-r-- 1 fernando fernando 773983 set 19 19:48 0d2ac266f98f416d9395f4a02b350c90.odt

And here is the file changed in the first machine, mas never brought to the second machine:

-rw-rw-r-- 1 fernando fernando 773715 set 19 20:12 0d2ac266f98f416d9395f4a02b350c90.odt

There are no errors registered on the log files on either machine.