5hours should allow you to do ~120x5=600 synths. Even at 60+ you're bound to gain 1 complete synth skill after 3-6dozen tries, so you must have been using the wrong recipes, or were expecting to skillup by desynthing (which rarely gets you any skillups).

I do wonder how widespread this thing really is, though. It's not an announced STT mass-ban, and as of last night I don't recall seeing any major thread on Nejitsu (the 2ch board where pretty much everything FF11 is covered).