Photos blurry after editing in Photos for Mac and no thumbnail

You've tried all the standard fixes so far. Those left are:

1 - download and install the macOS Catalina 10.15.5 Combo Update  

3 - boot into the Recovery volume (boot with the Command + R keys held down), select Disk Utility and repair the disk.

3 - if #2 doesn't help boot again into the Recovery volume (boot with the Command + R keys held down) and reinstall the system.

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.

The combo includes all of the incremental updates from 10.15.1 to 10.15.5. If there's a damaged file from one of the previous updates or an incremental update was skipped the combo will fill in the gaps and replace bad files.

I never let the System/Software Update do the updating automatically and always use the latest combo updater from the Apple Downloads page.

Have you run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac?

You have some adware installed. You can remove it via Etrecheck or download and run the free version of Malwarebytes. It was developed by a long time contributor to these forums and a highly respected member of the computer security community. 

One thing I didn't suggest is to log into another admin user account, Set up users, guests, and groups on Mac,  and see if the problem persists. (This tells us if the problem is limited to your user account or is system wide).

Boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and test there to see if the problem persists.  Reboot normally and test again. (Since no 3rd party apps can launch automatically in Safe Mode this tell us if the culprit might be some 3rd party software)

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

I do use the "Dr Cleaner" app, which has an optimise function - which I use frequently to free up working memory

Does this cause problems?

I believe the problems is tied in with the model Mac you have. I'm sure Apple has been made aware of it by other users but it won't hurt to keep letting them know via Feedback - Photos - Apple

I have this exact same issue - word for word.

I have tried the same fixes as you but no luck at all.

I am also running macOS Catalina version 10.15.5 (latest) and Photos version 5.0 (151.19.150)

Sorry this isn't a more helpful post, but I am keen to see if any fix is posted.

Likewise, I have recently started to experience the exact. same. issue, too.

You described it perfectly.

Photos is an unholy mess these days, bug after aggravating bug, and this is just the latest, and it will never be acknowledged, and will maybe quietly be fixed months from now.

But at least we're gonna get new emojis, right people?!?!

For what it's worth I tried the combo update as recommended above but sadly this doesn't solve the issue.

Photos really a pain, with thousands of pounds worth of kit you'd hope Apple's own software would turn smoothly.

Will try to repair disk in disk utility

Yes. There is no reason to ever run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac.  This user tip describes what you need to know and do to protect your Mac: Effective defenses against malware and other threats - Apple Community.  Uninstall it according to the developer's instructions. ; 

Hi Old Toad - thanks for the tip, I have done a full uninstall of DrCleaner and all associated files. I did a full reboot afterwards but unfortunately the issue in Photos persists unchanged.

When I edit a photo (either details or cropping) it shows the tile as greyed out in album view, and when clocking on that particular image, it never loads and remains blurry, nor will it let you share the photo. When I tried to share immediately after making an edit I received the error below:

Thanks for your assistance with this by the way. Photos has been a real nightmare lately.

Best