While Lenovo suggested doing a factory-reset (which always is the last-resort you get by support hotlines before they have you sent-in your device), this would have been easier to resolve (without data loss) – provided either your device is rooted, or you have a custom recovery available:
Whenever you switch to a different ROM, or upgrade/downgrade to a different Android version, it is recommended to wipe your cache and Dalvik/ART cache. Background is that different ROMs/Android-versions use different versions of the Dalvik/ART engine which might have (subtle or not-so-subtle) difference in structures – so when the "new version" accesses structures of the "previous version" which it doesn't understand, it leads to problems as described in the question.
For OTA updates provided "the official way", the responsibility to take care for that lies in the vicinity of the provider: Their update script(s) running on a level having access to both, and thus should take care to wipe cache as well as Dalvik/ART cache when needed (so blame them if they didn't).
If you flash a custom ROM, this task lies in your vicinity – as the "ROM cook" cannot know in advance if you're flashing it over a different ROM/version. Installation instructions for those custom ROMs usually include that step.
If you happen to be in the situation as described in the question, but don't want to root your device or install a custom recovery, you can either perform a factory-reset (WARNING: all your data and the apps you've installed are lost then, so take care having a good backup at hand) – or you can *boot (not install) a custom recovery once to wipe cache and Dalvik/ART. One recovery usually supporting that is twrp:
There's a guide to install TWRP on the Lenovo A2010 which includes a link to download TWRP for this device (Note: I cannot vouch for the safety of those sites or integrity of the download – so better check yourself for a reliable source, or verify this one).
Once you have the TWRP image file, you also need the fastboot tool on your computer. Boot your device into bootloader mode, then run
fastboot boot twrp.img (replace
twrp.img with the name of the
.img file you've got). Now your device will boot up into TWRP. Select to wipe cache and wipe Dalvik/ART, let it perform the action, then have it reboot normally. TWRP will be gone from your device, and everything should be "back to normal" – while no data have been lost.