I'm confused as to why my tags are missing after upgrading. I see two issues.

1. The tag information does not show up on the image page even though the template (I switched to using Basic) does have the call to display them
2. The tags I assigned are not checked for that image in the admin area. I note that the tag summary does show tag counts, however.
   I don't see any related php errors in the log.

Where do the tags come from? Did you set them manually on the backend or should they have been imported from metadata? For the latter you need to have the import enabled for that on the metadata options.

I'm not sure I follow... they were already created before the upgrade via the GUI (checkboxes) as tags, but they don't seem to be recognized after import.

Sorry, I was unclear probably. I meant should they have been checked via metadata import already or did you check them manually and they don't stick? (Just in case: don't forget to hit "apply" to save the selection ;-)).

They were already present in my old install before the upgrade. After the upgrade, the check boxes are no longer checked. However, the overall counts say that there are tags in the database -- they just don't seem to be associated with the images anymore.

Okay, I understand what you meant. The tags lost the assigment. The only way it could happen is that for some reason Zenphoto considered all albums/images new and therefore cleared the database on re-importing them freshly. Zenphoto being filesystem based syncs the database with the file system - the albums folder - and not the other way round. This can also happen if file/folder permissions are off and Zenphoto cannot read the files.

How did you upgrade exactly? Did you move the install on that occasion possibly? If you do that you need to striclty follow the order described on https://www.zenphoto.org/news/moving-and-changing-existing-installations/

If you have a database backup from before the upgrade you should try to use that. If you are from 1.6.8 now it should really be 1.6.7 or at least some 1.6.x. Older backups like 1.5.x or older may not work due to database changes (which I don't remember all anymore).

So I upgraded from 1.4.14 which may be the problem then if there were database changes. I did a database backup before upgrading so I could restore to that version and upgrade in smaller steps, but I don't want to do every single version since 1.4.14 or I would be at it all day... Everything else appears to have upgraded successfully. I skimmed the notes for the releases since 1.4.14 and it seemed they were minor changes or bugfixes from the news notes.
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I see that 1.6 had some structural changes. Would I have better chance for success if I upgraded to 1.5.9 from 1.4.14, then 1.6.0, and finally 1.6.8?

Actually, there is one more issue I've noticed: some admin pages don't seem to record the changes to the settings, even though a successful notification shows - for example, the contact form extension page.

Hello,
Probably a cache problem.
You can try to disable it and see if this soves your problem.

So I upgraded from 1.4.14 which may be the problem then if there were database changes. I did a database backup before upgrading so I could restore to that version and upgrade in smaller steps, but I don't want to do every single version since 1.4.14 or I would be at it all day..

Well, there is a reason we don't recommend to do directly upgrades from much older versions ;-) I did successfully previously myself even from older but there is no guarantee. You should not need to use really all versions, but try at least the main ones: 1.4.14 > 1.5 > 1.5.9 (last 1.5.x) > 1.6 > 1.6.8 (last 1.6.8)

Probably a cache problem. You can try to disable it and see if this soves your problem.

No, if settings don't stick this is not (page) cache issue. It's rather a database issue. For some reason some people sometiems loose the unique indexes on the table so options are frequently created as duplicates. Which means there are multiple and they are never really used. Never encountered this anywhere myself though.

It seems there is at least something wrong with the contact form regarding the text and math quiz fields. We'll investigate that…

It seems there is at least something wrong with the contact form regarding the text and math quiz fields. We'll investigate that…

In case you were referring to those specifially: They only show up if you are not loggedin since known users don't need to answer them. We were not aware of that anymore ourselves I admit…