I have tried to merge several photos in PSE 2020 and failed. Sometimes it will merge three out of the five and sometimes it just fails. Fortunately, I still have PSE 2014 on my computer and it works well as always. Yes, I convert all to jpeg and save them beforehand. Anyone have any suggestions?
2014 works but not 2020?
That is a question for Adobe support.
Anyhow, I would check the optional settings, comparing both programs, just in case something is different. I would check the temp location first.
Then I would check if there is any hardware acceleration disable in one (or enabled for that matter).
Basically the idea here is to use the 2014 settings and apply them to the 2020 version.
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