if PLC is running or not has nothing to do with the app, I guess this is a coincidence

it's even the other way around: if PLC is connected at the same time connection dropouts may occur when app and PLC try to communicate at the same time

-> there is something else wrong

check the usual things:
- IP (resp. domain name)
- password

-> all to be find under the system settings in ProfiLuxControl (Buttons "Webserver" and "LAN")

if you can't solve this then post your relevant settings of the app and in ProfiLuxControl