QuickBooks Online uses browser cache and cookies to load the pages faster. At times, these accumulated files become corrupted causing strange behaviors in the program. To rule this out, you can open your QuickBooks company in a private window. Here are the shortcut keys:
Ctrl + Shift + N for Google Chrome
Ctrl + Shift + P for Firefox & Microsoft Edge
Command + Shift + N for Safari
If you're now able to open your transactions, go back to their regular browser and then, clear its cache to delete the accumulated junk files and improve your browsing experience. Alternatively, you can switch to other supported web browsers to further isolate the issue.
For tips in managing your vendor and customer transactions, you can browse a topic from these links: