email - DKIM FAIL with domain null in GMail

I'm getting a DKIM: FAIL with domain null error in GMail. When I go to See Original in GMail, I get dkim=fail header.i=@example.com;.

In my DNS I have the following TXT Records:

DMARC TXT Entry: _dmarc v=DMARC1;p=reject;sp=reject;rf=afrf;pct=100;ruf=mailto:authfail@example.com;rua=mailto:mailreport@example.com;ri=86400

SPF TXT Entry: example.com

v=spf1 +a +mx +ip4:70.xxx.xxx.xxx +ip4:71.xxx.xxx.xxx +ip4:72.xxx.xxx.xxx ~all

DKIM TXT Entry: _default._domainkey

v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAo/HYr74H9Ty6HjTpjawYIfiDa7kOPx+0GJiw4rw7IYkfvrfrLojmGLuS4lg3NbIqYmLg9Ziz77q9vVT15ftrFB1+lTz6+Ei19b6AM8FGCJoDWRHRwB+YN0InJaFs/kPfy1SktqQI8TntPNfH2+tTOUrIT1gi8fjwSnfSkUsbNuSpIp3XrAU6pYzEjXF2h1A3UMFV+7+jLRzEvunj2YCsJ3vaifTihat+Nsw7eP5L2iAzW8b0joP3z6k/UkhUa7/35oNC4Xn1dY532irsBunMJz9QsBxIaNpepiZZkTxfglsq2RP4FWGZbiVFRDDcvsnpi4kFx3eAQtAoo+szLA70FQIDAQAB

I tested my records via https://www.mail-tester.com/spf-dkim-check and http://dkimcore.org/tools/keycheck.html and it all checks out. Public Key is 2048-bit. I read the GMail was failing DKIM with smaller keys.

I also tested it with http://unlocktheinbox.com and at the DKIM Validation Check it says:

Signature Found:    Yes
SmarterMail DKIM Test:  Failed - Bad Signature
MailBee.NET DKIM Test:  Failed - SignatureInvalid

GMail doesn't seem to have any issues with my SPF and DMARC. Here's the authentication-results for those:

spf=pass (google.com: domain of sender@example.com designates 70.xxx.xxx.xxx as permitted sender) smtp.mailfrom=sender@example.com;



dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=example.com

My emails are hosted/sent from http://inmotionhosting.com. My DNS is at CloudFlare.

What am I doing wrong or missing here?

Thanks!