Yeah, like mustang I was wondering what app it was. Like B. said if the app dev isn't helping or doing any more with the app replacing it may be the only work around.
Given the app isn't working for you anyway, I can think of one other thing to try but you might consider it risky. I've seen plenty of apps that are not compatible with newer Android versions in which case the Play Store will not offer them when searched from a device they are not compatible with, often not even displaying them in search results. I can come up with a dozen reasons for the app to be buggy after an update or glitch after one especially if it's heavily used. Let's do some basic stuff first.
Have you cleared the apps cache? You can do this manually from the app page in your settings. Settings>Applications>app x (we don't know the name)>Storage>Clear cache, you could also try clearing the app data but depending on the app you might lose other saved data, which isn't too bad if it's stored in the cloud or in an account associated with the app. So hold off on clearing the data for now.
What was the result? if it didn't work you might try clearing the device's Partition Cache. You don't tell us what device you have so specific instructions are not available. Usually this is done by turning on the device using some combination of button presses to enter the Bootloader, selecting clear partition or partition cache and restarting the phone. This will not cause you to lose anything but after a big update there can be stuff left here that causes bugs and issues. This isn't a bad idea after any big update and is a good idea if things are noticed to be wrong afterwards.
If both of those fail now would be the time to try clearing the app data. At this point you have little to lose. Following this you might try reinstalling the app. After clearing the data you've essentially started over with the app so uninstalling then reinstalling it just means that you have the most up to date and freshest version possible. Back to the risk I spoke of earlier, if the app truly is no longer compatible with the device it may not show up in search results making it difficult or impossible to reinstall. Which is why I put at the end of this list.
Best wishes.
Let us know what device you have and what the app is and we'll offer more advice.