Do you think it might be a safety thing? People opening up a roof at say 50mph could easily cause that roof to become a sail and either be ripped off causing a hazard of flying debris to other traffic or the sudden drag of that sail could cause the car to swerve.
If this has only happened after a service and they updated the software during that service, i'd suggest this is a very likely option.
However, it would make more sense for the roof to only be operable when the engine is running (so the alternator is charging the battery) but the car is stationary. It may take the stationary signal from a wheel speed sensor (usually used for ABS and maybe traction control and similar) and that sensor is faulty causing the roof to not operate - have you consulted your handbook as to why the button flashes? A flashing button usually indicates a fault in the system somewhere so i'd suggest that as the second most likely.
***EDIT***
https://az685612.vo.msecnd.net/pdfs/..._EN_tp8337.pdf
Scroll down to Pg75 of that if you don't have an owners manual, granted it's for 2006 but Volvo don't seem to have anything older on their support site. Are you pressing the brake pedal while trying to open the roof with the engine running? That may help and also if you have any brake light bulbs out or the brake pedal switch isn't working/brake light fuse failed, it may not make any difference to the button flashing.