I think I am in the minority in that I see by far the most likely culprit as internal. For something like this I think you need 3 things:
1)the means to counterfeit the bills - you need printing presses capable of creating large numbers of IBC's that look just as good as normal IBC's without it being immediately noticeable due to a lack of book/manuscript production
- this I believe disqualifies Wallachia, Russia, Georgia, Ottomans, North Italy, and Germany. None of them would have the spare capacity to create large numbers of bills without it being very noticeable though for different reasons.
-Wallachia, Russia, Georgia, Ethiopia, Oman, Serbia wouldn't have the capacity as they don't feature large literate populations that would justify large numbers of printing presses even existing let alone such a large number that a couple dozen can be re-purposed without it causing a massive drop in book production that would be noticeable to everyone including any resident Roman ambassadors
-I think the Ottomans are similar to the above in that they don't have large literate populations to justify and if it follows a similar path to OTL the literate populations will place more emphasis on the aesthetics of the book and thus prefer hand-written over machine produced (IOTL the first Muslim owned printing press was established in 1727 though non-Muslim printing presses did produce Arabic works)
- Germany and North Italy are both currently warzones and the belief that someone could spare the resources for a sophisticated counterfeiting scheme strains plausibility though by no means would it be impossible
2)the motive to counterfeit the bills - there needs to be some reason why this would be done that isn't just cartoon villiany "because I can". This would more or less eliminate the entire Orthodox world since all of them are reliant to an extent on the trade and banking of Rhomania and the thought that any Orthodox government would risk that seems insane.
3) the ability to distribute the bills fairly evenly across the realm - perhaps the most difficult part that I think disqualifies every single foreign actor.
- if for example large numbers of IBC's kept pouring out of the Latin Trading quarters and specifically those where Triunes or Spain operated it wouldn't take over a year and a secret investigation to figure out. It would be plainly obvious that the Latin's are sending out fake ones and the solution would be obvious. Invalidate all IBC's held by Latin's and bar Latin traders from using anything but bullion.
No foreign power can do all 3 things. The orthodox powers could do (3) but not (1) and (2). Ethiopia arguably could accomplish (2) and (3) but (1) is a big sticking point. Triunes, Arles and Spain are the Latin powers I could see fulfilling (1) and (2) but would not be able to accomplish (3). The Ottomans definitely fulfill (2) but (1) is very arguable and (3) would be a non-starter.
On the other hand a group of domestic dynatoi in league with a faction of the banking board could easily fulfill all 3 criteria:
1) A group of dynatoi with a faction of the reserve bank board would have access to printing presses to manufacture them and Rhomania has sooo many printing presses that 15 or 20 not producing books would not be noticeable especially if the dynatoi also invested in a couple new printing presses. As well the bank faction could provide legitimate dyes so for all practical purposes the fake IBC's would be real ones.
2) This group would have various motivations: for the bankers themselves they firmly believe a ratio of 5:1 or 6:1 or greater is achievable and this is their way to "prove" it. For the Dynatoi it gives them access to a parallel vehicle of wealth that is immune to Constantinople's many laws that target the dynatoi; the most recent of which is progressive taxation; since the government does not know it exists. If they could have kept the ruse going even 10 more years they might have been able to amass enough power and wealth that the relationship between the dynatoi and empire would more closely resemble the 11th and 12th centuries when they held real power.
3) Between the bankers using bank branches and the dynatoi filtering the IBC's through their scattered holdings there wouldn't be a concentration of IBC's that would tip everyone off that something was wrong right away. Also even when people realize something is wrong it is not clear where the problem originates since it doesn't originate at any one place. It was everywhere at more or less the same time.
