Impossible to open the apk file once downloaded

Good evening and thank you both for your responsiveness :)

I have already tried apk installer and I also receive an error message when I attempt to install the .apk downloaded from the internet (I indeed have no issue with installing apps via the Play Store): the message is "no activity found to handle intent"

However, I tried framaroot and towelroot but they do not work with the latest Android versions (KitKat 4.4.2) like most rooting apps; only king root worked (I have king user and super user, as well as a busybox that I am unable to use... anyway)

I want to specify that the apps in question are not corrupted. Specifically, it is pokerstarsinstallfr.apk which worked very well before.
I tried with another app downloaded via 4shared, but the problem is the same, so I deduce that I must have lost the installation rights via the ROM when I rooted, as I have certainly checked the permission to download from unknown sources. Since my knowledge is limited, I find it difficult to know how to resolve this or how to make a backup. I tried, for example, to stop the root temporarily by disabling superuser and kinguser, but I am not even sure I can stop them.

For your information, I have titanium backup, which allows me to manage my apps, since this no longer works via the Android ROM menu.
Could it be that it is blocking?
Historically, I had installed the pokerstars apk before rooting. Download ok. Install ok. Use ok.
Then I rooted and installed titanium backup. A few weeks later, the pokerstars app that was working very well needed an update.
It would launch but in the end, the pokerstars app crashed, "the app xxx has stopped" (it's the same message when I try to delete an app via the Android menu). So I uninstalled and reinstalled but "unable to open the file" that I was able to open a few weeks ago

I hesitate to delete Titanium backup because I’m afraid I will no longer have control over the deletion of apps and lose the backups of system apps that I had to disable to gain battery autonomy (it's incredible what one has to do to have a functional tablet with the new Android versions: the battery being drained by searching for a GSM signal when there is no SIM inserted and no deactivation option without root; the storage space on the SD being unavailable even with root)

Sorry for this long development but maybe my experience can help others.

The best way to root a phone or tablet is to use Framaroot or Towelroot v1. Then, remember to install SuperSU, which controls the applications that want to access root and offers you the choice to accept or deny an application's request for root access.

Next, I would like to know if you downloaded the application via the Play Store or through the internet?

Best regards