What follows is strictly my opinion re use of "registry tools" and rambling about in the registry.
IMO, most knowledgeable members don't use registry tools to make changes...a knowledgeable person will make registry edits manually, targeting specific keys and values...and do so confident in the knowledge that he/she can reverse or undo any changes which he/she may make. The analogy that I might use is that of a craftsman...who has a clear idea of what he/she wants to do and how to go about trying to achieve the desired result, based on plans, documentation, and experience.
Anything which a "registry cleaner/optimizer" does...amounts to a shotgun blast, according to some template worked out who knows when...that is applied to every computer system on which said software is run. The template may relate to some general concept of what a system registry should look like...it does not/cannot relate to the specific registry of a given system...since these vary according to what is installed on the system. The registry for a system with current critical updates installed...must vary considerably from that of a system without such. The registry for a system with an OEM (HP, Lenovo, etc.) manufacturer...may vary from that of a person who installed a version of Windows which does not have manufacturer's own installed programs/partitions/backup or restore-to-factory defaults mechanisms. And so on.
I could never assume...that the theoretical "fixes" applied by a registry cleaner...are ones which need to be made to overcome my specific problem...unless I knew what those "fixes" are...and I knew what specific problems they are designed to overcome.
Most users of registry tools fail this test. They are groping blindly in the dark for a "solution" to that which they may construe as problematical...or some shortcut to "better performance." Rather than take the time to try to properly troubleshoot known issues, these users want a "quick fix", a "deus ex machina" to save them the time and effort it takes to try to achieve what they want.
There is no universal panacea for all computer system problems, due to the nature of the computing. Various program developers take advantage of the lack of knowledge by some computer users...and spew forth "solutions" for that market.
Re Ccleaner...it has a registry edit function as part of the package of what it purports to do. I've never found that Ccleaner does anything I cannot do myself, as far as the other functions I have seen detailed. Windows itself provides the capability to do most, if not all, else that Ccleaner advertises.
As for the claim of removing "unused and old registry entries"...there is no need at all to do that. And, if a user feels that he/she wants to do it...such can be done manually. I must add...Windows installs a number of registry entries to facilitate installing/using various programs...perhaps these are part/all of the "unused" category referred to. My basic problem with any product advertising such a dubious benefit is that I have no clue how Ccleaner determines what is "old" or "unused" and I have no idea why it wants to remove such entries.
I could go on, but I think you get the picture.
FWIW: I had a system dating back to Win 95, I made no effort at all to find out what the registry is until I had moved to Win 98. That is when a number of these "registry cleaners/optimizers" first appeared, with Norton Utilities being one of the first ones that people employed. As more data began to become accumulated about the performance/value of such programs...it became clear that these programs often caused problems with systems.
These programs evolved out of a sitution where new users did not know very much about Windows and/or a system...and, in the face of the unknown, they sought easy answers.
As far as I am concerned, there are no easy answers to system problems. Users who are unwilling to do the research and troubleshooting that MAY lead to a solution...will forever be subject to exploitation and bad advice/suggestions that are often intended to help.
The computer forums which exist today...and Google...are two of the best tools for any user who wants to come out of the dark...into the light.
I don't intend to dissuade from using CCleaner, Advanced System Care, etc...that's a user decision. But the website tried to let such users know that there potential problems with groping around in the dark and believing rhetoric which is clearly designed for the uninformed, unknowing user.
The BC statement is designed to try to remove some of that lack of knowledge, no more, no less, IMO.
Another FWIW: The best way to "speed up" your system is...install max RAM supported, ensure that Windows has the suggested minimum amount of free space on the Windows partition, eliminate programs which are fluff (consume system resources without providing any real value), and perform the basic routine maintenance on Windows.
As I see it
<<First of all, why have the errors stopped? Is this believed to be complete coincidence, has it actually fixed the error but done so improperly, or does the possibility exist that this particular time it fixed the error and did so without harmful side effects?>>
I cannot answer that, since I have no clue as to what was done. I can't speculate on what transpired, but I would not be surpriesed if you have problems post your "problem-elimination."