Sometimes driver trouble gets in the way of MTP functioning properly … this here should help (if driver trouble would really be the issue) …
Disconnect the phone from the computer.
In Control Panel - Programs and Features:
Uninstall everything with “MTP” or “ADB” in its name. And the “Android SDK”, if it is installed.
In Control Panel - Device Manager:
Enable View - Show hidden devices, and then uninstall every possible ADB, MTP and smartphone device, hidden or not.
Connect the phone again
Windows should then install working MTP drivers again automatically.
If this doesn’t work, the problem is probably on the phone’s side (*).
If you had adb and/or fastboot before, they might have gotten uninstalled now if they originated from a formerly installed package like “Minimal ADB and Fastboot” or something similar, so (and it’s better to use the following anyway) …
(*) At least if MTP access to the phone worked before. Just as a reference … If there would have been a problem with MTP from the start, it could have been due to a Windows N version. (“N” = without Media Player, result of an old antitrust case)
A Windows N version would need the “Media Feature Pack” before it can do MTP. This should be installable via Settings - Apps - Apps & Features - Optional features (or something like that, German locale here) in current Windows 10 N, for older Windows N versions there were dedicated downloads … https://support.microsoft.com/en-us/help/3145500/media-feature-pack-list-for-windows-n-editions
Yeah, sorry, I forgot to write that I found this solution in the forum and tried it already, but it didn’t solve the problem unfortunately. There weren’t any “MTP” or “ADB” programs to uninstall and uninstalling all previous smartphone devices in the Device Manager didn’t solve it either.
I followed your instructions and added the path to the SDK platform tools. Should I run the program from somewhere?