I have been running Zenphoto for a few years now with no issues. Recently my latest two folders (spanning the last two years) won't display. All other folders display fine, but the page where my recent pictures should display is showing a blank where the thumbnails ought to be and the Admin link is gone too. If I move these two folders out of the albums directory then ZenPhoto looks great and doesn't appear to have any problems. When I move those folders back in it breaks the page I would expect to see them on. It's like the photos are corrupted. But they were all working fine until just recently. As far as I know, nothing has changed on the server or in this installation. I'm seeing things like this in the logs:
WARNING: Illegal string offset 'SubjectDistance' in /var/www/html/photo/zp-core/exif/makers/canon.php on line 319

formatCanonData called from parseCanon (canon.php [408])

from read_entry (exif.php [768])

from read_exif_data_raw (exif.php [1028])

from read_exif_data_protected (functions.php [2210])

from Image->updateMetaData (class-image.php [335])

from Image->__construct (class-image.php [156])

from newImage (class-image.php [52])

from uploader.php [99]

{28112:Wed, 30 Jan 2019 03:25:51 GMT}

EXCEPTION (0): Cannot use assign-op operators with string offsets in /var/www/html/photo/zp-core/exif/makers/canon.php on line 319

Permissions are the same throughout my folders and I can see nothing obviously different between the pictures I can see and the ones I can't. I wonder if there is something obvious I'm missing; any insight would be much appreciated

I assume you are on the latest 1.5.1 release. If not please consider an update (if you are on an much older version and use custom theme etc. this might be some work).

I don't think anything changed in meta data handling. So the folders did exist already two years and it suddenly happened? Has the server been upgraded recently (PHP especially?).

As far as I can tell you're still on the 1.4.13 release.
Meanwhile a similar issue has been fixed.
Please upgrade to the latest release (1.5.1)

I recently had my folders stop appearing. Turned out that the 'published' buttons in the control panel spontaneously turned off. Looking around the administration panels showed this state. Turning them back on made them reappear.

It would be interesting to know how this can happen sponantously without any action, either clicking or using the bulk action.

Do you use the publish content plugin and perhaps accidentally set a default of not publishing new albums?

I hadn't done anything in the control panel for quite some time. My first assumption was that something had taken place on the commercial web host's server.

That of course might be like a database backup. But that should not change any database values (publish status is just 0 or 1).