I have gmail account signed in and when I open google forms that I received in my inbox, it asks me to sign in to continue. When I click sign in, it again reloads the same page and again asks me sign in to continue.
i know this is an old post but this answer is for anyone else looking for the same solution to a similar problem quite recently.
Google drive & its different drive products access is limited or granted to each users google account based on the access permissions that the user has to each google product, in this case forms is part of the google docs product category under the google drive family- you can only access your google drive folder or products or those that have been shared with you only if you are already logged in into your own google id - whether personal or gsuit does not matter - but you need to have access permission granted to the files you want to access - this is accomplished by your google login id & password.
this sometimes get messed up when there are multiple google accounts being used on the same desktop browser or device and google is unable to authenticate the credintials of the correct user of its google drive products due to multiple login credintials, cookies etc.
Hence this works in incognito mode cause incognito mode or simple safe mode with no addons etc as offered by many popular browsers -does not retain any login credintial backend data like google accounts login cache or cookies or certificates data etc etc.
So when you login on incognito or safe mode, the browser has no record of your gmail account login details whatsoever, other than your password in the password manager auto fill (for example- the google chrome password manager) - it logs in without any issue.
for example google chrome uses the first main users login details as the primary account on the chrome browser, yes you can add multiple accounts but it still considers the first account as the primary account, although you can always change that from the settings page, sometimes when users add their login details to the browser they click on stay "always logged in" for example say its a home shared pc, like you there may be others who may have saved their login info on that shared family pc- this is where the mess up occurs at times, but the fix is very simple.
A simple workaround to this issue is :
logout "all" google accounts from the desktop browser.
Clear the cookies & cache
Close the browser completly
Reopen the browser &Try to login in again -with your google account as the primary account (like on google chrome) it will work just fine
This solution can also be used when you are unable to enter the google script editor from the google sheets menue - access is denied with an error code 'you can access the script editor at this time ' the problem is caused by the same reasons mentioned above, mixed up google login credintals.