App won't install on android.

Not wanting to factory reset, I removed the SD card. I restarted the phone which performed the updates that were causing issues. Then I put the SD card back in. Since then, everything is working.
Ch@pi.

http://www.forum-smartphone.com/forum/probleme-d-application-help-t811.html
while doing some research, I found a good number of people who apparently have the same problem; for some, a simple factory reset fixed the issue, otherwise it's a manufacturing defect :D

If the manipulation is done on another phone, unless otherwise indicated, I advise you not to redo it on your phone. You can try analyzing your SD card from your computer, in case it contains errors. Otherwise, have you already tried reinstalling an application to see if it would work again? If there are no errors at the save location or in the application itself, since the problem is the same with all your applications, it could be your phone that has an issue. If it's not a misconfiguration, the only solution will be to send it for repair. So first, try deleting an app and then reinstalling it to see if it works. As a last resort, if it still doesn't work, you can always reset your phone to factory settings.

Thank you, but I have space on my phone's memory, and I have 3GB free on my SD card, and it still won't install, even though I've moved almost all my apps to the SD card.
And when the market tells me that there's an update for an app that's on the SD card, I do it, but at the time of installation it shows me installation failed.
I don't understand.
Also, I saw on a site that deleting a folder would help, but the method is done on a HTC Desire S, and I don't know if it would work on my Samsung Galaxy Ace.
I'll put the link to the site: https://forums.cnetfrance.fr/tutoriels-smartphones-et-android/293599-android-contourner-l-erreur-unknown-reason-18-pour-installer-une-application
Thank you

An app doesn’t take up much space, so manage your memory; you may have filled it with videos or music. For those that won’t update, go to the Android Market and click on the "My Apps" tab, which will show you the apps that need manual updates or not. Tap to update. If the apps won’t install and it’s not due to memory, they may not be electronically signed; therefore, your phone doesn’t recognize them and won’t install them.