Thanks so much for your thoughts and advice! Interestingly enough, I decided to run both the "scannow" and "chkdsk" processes, as was suggested by mta. Scannow successfully ran and uncovered zero issues. Chkdsk said it required a reboot before it could execute, so I allowed it to do so. Unfortunately, it got stuck at 10% for about four hours, so I did a hard reset. When my PC tried to restart, it gave me a BSOD. I let it restart on its own, and it gave another BSOD. I didn't want to let it crash a third time, so I did another hard reset in hopes of breaking the loop. This time when it booted back up, it gave me a message saying that there was an error, and that Windows 10 would need to repair something. I let it run its course, which took about 1 hour. It automatically restarted after this, and it booted successfully with no BSODs or issues. I immediately re-ran "chkdsk" via Command Prompt. It didn't give me the original message regarding it requiring a restart to execute, so it ran its course and did not uncover any problems. Following that, I updated Defraggler to the newest version. When I opened it up... the warning message was gone, and it now displays "Good" in green, as it had before. Isn't that strange?
Anyhow, I share all of this for some closure for anyone who was curious what the outcome of this entire debacle was. Everything seems to now be fine, and even as of this morning Defraggler is still showing my HD health as being "Good" in green, so I don't know if the outdated version of Defraggler was the cause of concern or if there was something that genuinely needed to be repaired (or if the repair only came about because the chkdsk process had frozen and been cut off too early)... ahhhh, there's just no way to know.
I will be mindful of not defragging my entire PC moving forward, just in case this habit was somehow tied into the whole situation. If anything, I'll analyze my disk and try to only defrag selected files that are excessively fragmented.
Thanks for all of the advice, everyone! I'll be sure to keep this thread posted/updated if the "Warning" message returns.