Thanks a lot Goran for your kind advice which I'll certanly follow,
however, my Avira is the Pro payware version that presently covers two Windows PC's plus three Android hardware and has been there since several years never causing any kind of problem. As far as XPlane11 is concerned, installed addons such as Zibo737, Rotate-MD80, JRollon-SF260, Airfoillabs-C172SP did not suffer similar difficulties. Avira may be a junk as Cameron states and I'll get rid of it as he suggests, I'm not qualified at all to judge, but it's quite pricey and widely adopted and a lot of TDM900 buyers, particularly in Europe, might have Avira installed and potentially bother you and waste your precious time as I did. Therefore I think that investigation/solution about the TBM900-Avira conflict would benefit both your Customers and You. Your suggestion for a replacement good payware antivirus compatible with TBM900 will be mostly appreciated !In the meantime, not being an expert, I'll try to make Avira not scan the XP11 Folder....