For some considerable time it has been the plan to place limits on the total number of media items that each member can upload and store in their media centre and these limits vary by subscription:
Free (no paid subscription) - 12
Silver - 250
Gold - 1000
Platinum - 10000
The above implemented 11th May.
These limits have been necessary to primarily enhance system performance since at the moment too much resource is being afforded to media storage and processing.
Members who have exceeded their respective media limit will have to clear some 'space' before they can upload new media items or upgrade if that works for them as a solution. This does not affect media that is added using the 'External' option to messages, forum threads, group posts etc. since they are not saved in your media centre.
Be aware that in calculating your total media content its the figure you see on your profile page against media plus the entries in your Misc album.