I have a Google Apps Script program that Does Stuff to Trello cards (and other things) based on external outputs. In this case, we cycle through cards in a certain lane and download their attachments to Google Drive folders indicated in a custom field on each card, then mark the card to say the files have been downloaded. Pretty simple stuff, except Trello's API now doesn't let you directly download files without authentication. While most of the operations you can do in the API just let you pass key and token as URL parameters, downloading a file makes you set up Oauth1 to initiate the GET request. And for the life of me I can't get it to work. I'm using the GAS Oauth1 library from here:

https://github.com/googleworkspace/apps-script-oauth1

And the Trello sample for the library from here:

https://github.com/googleworkspace/apps-script-oauth1/blob/master/samples/Trello.gs

When I run the code, I get "Error starting OAuth flow: Invalid Signature." Here's the contents of the connection object:

{authorizationUrl_=https://trello.com/1/OAuthAuthorizeToken?scope=read,write, 
accessTokenUrl_=https://trello.com/1/OAuthGetAccessToken, 
consumerSecret_=[REDACTED], 
paramLocation_=auth-header, 
requestTokenUrl_=https://trello.com/1/OAuthGetRequestToken, 
scriptId_=[REDACTED], 
consumerKey_=[REDACTED],
method_=get, 
serviceName_=trello, 
propertyStore_=ScriptProperties, 
signatureMethod_=HMAC-SHA1, 
callbackFunctionName_=authCallback_Trello, 
oauthVersion_=1.0a}

It's not the library, because I can authenticate and post to Twitter through OAuth fine using the same library from the same GAS project. But the error is so vague I can't diagnose what the issue might be. I've tried a variety of different connection scripts but can't get any closer to figuring it out.