Hi Gunther, as I said if you post anything to BLP or anywhere else on the web (so the image is viewed from the internet via a web browser), it's almost essential to convert to sRGB as just about every browser out there assumes that's what's being used. Adobe RGB is a wider colour space and useful for editing or viewing in software that supports that colour space. By all means, keep all your files as Adobe RGB but whenever it leaves your computer for the internet, make sure it's in sRGB colour space. Also, watch out if you ever get your photos printed. Professional printing labs will tell you what colour space to provide your photo in. If you don't use that, it's likely your photo's colours won't print correctly. The default, again, is sRGB and it's likely most consumer-level printing services will expect a file to be in that colour space.
One thing that should not happen though is that the photo will look different when converted to sRGB (except in quite subtle ways). If you are using Photoshop or any other reputable image editing software, the software will look at the colour space metadata tag of your image and render it correctly no matter what colour space you use. So a photo in Adobe RGB should look pretty well the same when converted to sRGB in your image software. You indicate you converted to sRGB then the colour cast appeared; well, that should not happen. The problem only happens when the viewing application (like a web browser) thinks the colour space is sRGB when it's actually something else like Adobe RGB since the individual RGB values of each pixel map to somewhat different colours in the two different colour spaces.. So I'm not sure why you are seeing what you are seeing.