Hello, thank you all for your help !
The log file is a bit complicated to share as it contains some private links (nothing abnormal don’t worry ^^) among many many others, so it is difficult to sort them before showing it.
What is strange, is that since Firefox 64.0 update this morning, I have no more issue (it shows all good - Last synchronized : 3 mn ago). I really don’t know what happened as it is the first time I install completely floccus. The only change was Firefox update (for your information, I am on Linux).
I keep this thread open a few days, and if there is no more issue, I will close it.
EDIT : I confirm that since the new Firefox version, I have no more trouble.