My work is currently migrating from a fully on-prem environment to Microsoft 365 and SharePoint. However, due to various reasons, we've had to keep our on-prem domain active. Our domain controller runs on Windows SBS 2011 (Yes, I am aware that this is very, very old software, but the budget hasn't been there to upgrade it). Therefore, we have a virtual machine running Server 2012 that runs AD Sync. We have another SBS 2011 server that hosts our network shares.
All of our computers, aside from the servers, have been AD joined. We are having an issue now where some users, seemingly at random, are trying to access network shares, however when prompted to enter for their credentials, it refuses them even when they are correct. After a couple tries, their account gets locked out. I sometimes have luck by having them reset their password, however this does not always work.
Any ideas as to why this may be happening? I suspect it's just because of how messy our network is now; with the old software, hybrid environment, and general poor design and management of the network by previous IT guys. Very spit-and-duct-taped if you know what I mean.