If you have multiple Oracle clients installed but only need one, remove the one(s) that you no longer use.
If you have an ORACLE_HOME environment variable set, make sure that it is correct. (Or just remove it as it is unnecessary...this should be in registry for full clients and not needed otherwise).
Make sure that you have read and execute privileges on the OCI.DLL file in your Oracle client folder.
By the way, Toad 14.0 (due out this week) will include a troubleshooter for this exact problem, because the error message is so vague.
I ended up uninstalling the instant client and installing 12.1 64 bit regular client. The TNSnames are correct, i can ping the database, all prereq's passed, and all paths are correct. I chose run as administrator on the toad installer and again when i tried to log in.
But, I still get the can't initialize error. I've yet to see a fix for this. I know you said Toad 14 is releasing soon, but do you have any idea why this is happening?
The problem with Dynatrace happened because that software had a hook into the Windows API CreateFileA procedure. A component of Toad also hooks this procedure, and did not account for the possibility that some other software would be doing that also. Because the "other" software is 64 bit, we could work around this in Toad by installing the 32 bit version.
You said you don't have DynaTrace, but maybe you have some other 64-bit software that also hooks the CreateFileA procedure, and is causing the same problem in Toad. If so, you could work around it by removing your 64 bit Toad and Oracle client, and installing 32 bit Toad and Oracle client.
If that solves it, I'd be interested to know what other software you have installed on your PC.
I don't have a 32 bit version of TOAD. My PC is new because my old PC (which ran Toad just fine) died on me. The only other 64 bit program is carbon black which was just installed as well.
Click the link below and try the steps outlined there to remove all traces of Oracle, reboot, then reinstall:
If the problem persists after that, it is because of a CreateFileA conflict as I described above. Toad software licenses include 32 and 64 bit verisons. You, or whoever bought your license can download the 32 bit version here: https://support.quest.com/toad-for-oracle/13.3/download-new-releases
I installed 32 bit everything and still couldn't connect.
It was because the oraaccess.xml file was blank. I only figured that out because I tried to connect to the database from the command prompt. I typed sqlplus at the prompt add got the error that took me to the xml file.
I had to resurrect an old one (which I didn't even realize i had) and copy it to the directory where the 32 bit client lives and BOOM!
The only copy of it that I have is in the RDBMS\DEMO folder. I would think it doesn't get used at all in that location. Is yours in the TNS_ADMIN location?