When importing a Salesforce report, you may see there are 0 contacts pulled in, with a warning icon indicating:
You may be receiving this error if your Report does not include the Lead/Contact ID field which is required for a successful import into Act-On.
Reason for the Error
If your report does include a Lead/Contact ID field, this error message may be appearing because the Act-On user connecting to your Salesforce account needs to be set to Classic mode.
How to Resolve
Salesforce is currently making changes to Lightning and Classic access. Act-On is implementing a long-term solution to this issue, but for now you can resolve this error by disabling Lightning auto-switch by using a permission set. Make sure that your Act-On user that connects to Salesforce is part of this limited permission set.