FC Koln have confirmed that three members of staff have tested positive for coronavirus.
It's a blow for those hoping to resume the Bundesliga before the end of May.
The club say the entire squad was tested on Thursday, along with the backroom staff, and three people tested positive without showing any symptoms.
Koln did not reveal the identities of those involved, nor whether they are players or staff.
All three have now entered a 14-day quarantine period at their respective homes.
The German Football Federation (DFB) and the German Football League (DFL) are to present their restart plans to government officials on Wednesday (May 6).
Squads have already begun training in groups, and player testing got underway on Thursday.
Games will likely resume later this month behind closed doors, and with stadiums divided into three sectors into which only 100 people will be allowed.
However, the three positive tests at Koln show the precarious nature of the situation.
Koln's managing director Horst Heldt said on Friday, "The health and privacy of players and staff have priority above all else.
"The previous measures, as well as the strategy of regular tests, have proven themselves so that we can now react with individual solutions."
Koln training has continued as normal after the positive tests, "according to the hygiene and infection control measures that have been in place since April 6th in group training".
Prof. Tim Meyer is the medical director of the Institute for Sports and Preventive Medicine at Saarland University, and he added, "We now see in everyday life that our concept recognises and reduces risks at an early stage.
"We will always be in close contact with the responsible health authorities and medical experts.
"We are convinced that with our concept, we can enable players to practice their profession with the best possible protection against infection."