It looks like the statistic name is too long by just a few characters. The statistic name is composed of the population name and the statistic of interest. For example, the name could look something like "/S/Lv/L/CD3+/IFNg+:Count" or "/S/Lv/L/CD3+/IFNg+:Freq_Of_Parent". I vaguely remember FlowJo had a limit on the total population name length such that it would fit under our current limit, but they may have increased it in recent years.

For a workaround, you can try shortening the population names and re-importing into LabKey. I've filed an issue to address this:

https://www.labkey.org/home/Developer/issues/issues-details.view?issueId=43360