Updating to Ubuntu 20.04 partially broke my Windows mounts

I have an Ubuntu 18.04 server that I upgraded to 20.04 over the weekend (via do-release-upgrade).

This server has a permanent mount to a Windows server (I think it's Windows Server 2012, because it has that Windows 8-like interface).

Here's the /etc/fstab (The important line is third from the bottom, mounting //192.168.0.248/ArtDept/ArtDept/JOBS to /mnt/jobs_main):

# /etc/fstab: static file system information.## Use 'blkid' to print the universally unique identifier for a# device; this may be used with UUID= as a more robust way to name devices# that works even if disks are added and removed. See fstab(5).## <file system> <mount point>   <type>  <options>       <dump>  <pass>/dev/mapper/SkyUbuntu--vg-root /               ext4    errors=remount-ro 0       1# /boot was on /dev/sda1 during installationUUID=ef598c51-5b66-4749-a7be-da5c59ac852e /boot           ext2    defaults        0       2/dev/mapper/SkyUbuntu--vg-swap_1 none            swap    sw              0       0/dev/fd0        /media/floppy0  auto    rw,user,noauto,exec,utf8 0       0//192.168.0.248/ArtDept/ArtDept/JOBS    /mnt/jobs_main         cifs   credentials=/home/sturm/.smbcredentials,iocharset=utf8   0   0#//192.168.0.248/Jobs_Archive            /mnt/jobs_archive_01   cifs   credentials=/home/sturm/.smbcredentials,iocharset=utf8   0   0#//192.168.0.248/Archive2/Jobs_Archive   /mnt/jobs_archive_02   cifs   credentials=/home/sturm/.smbcredentials,iocharset=utf8   0   0

In the /home/sturm/.smbcredentials file:

username=<not>password=<gonna>domain=<show>

Before the upgrade, a mission-critical web app that runs on this server (Laravel-based, in a Docker container) displays a thumbnail .JPG that links to a .PDF, if those files exist. Otherwise, it just displays text without linking:

Unfortunately, after the upgrade, the thumbnails and links are broken:

Now, you may notice that the links are still there and the popup still appears. This is because the code that tests the existence of the files is still seeing that the files exist. In fact, just navigating in the OS to the mounts works fine. I can list the directories on the mounts with no problem at all. So the mounts are still working, but only to a degree. That degree being showing the .JPG files and linking to .PDF files in the web app.

I've tried various additions to the /etc/fstab file, such as adding vers=3.0, sec=ntlm, noperm, etc. Each time, running sudo umount -a followed by sudo mount -a. Nothing seems to have any effect.

I don't know what has changed with the CIFS mounting system between 18.04 and 20.04, but we really need this to work again. Any ideas?