Hello Max.
Welcome back to collecting...
1) Are there new error coins, such as the 2008 F 2€ Hamburg with old map?
As far as I know only:
EURO-BELGIUM-2014-2€ CC-"Red cross"-CoinCard, which has two "Error" variants. One with Dutch and one with Italian edge of coin.
2) Is euroswapper.info still the most popular swapping/selling site? Eurohobby seems inactive (didn't log in though)
For me yes. But it is the only web site I'm using...

Eurohobby was inactive for quite a long time. Think that it was sold few months ago...but I'm really not the most reliable person to give you info about Eurohobby. Maybe some other swapper will help.
3) Are Andorra's 2€ coins only issued in coincards and proof sets? Is it same for Belgium for most 2€ CC issued since 2014?
Andorra issued 2014-2019 2€ CC UNC coins only in coin cards. Think only 2014 was issued in PROOF.
Some of 2€ circulation coins were issued also in rolls. Think 2014, 2017 and 2018.
BU sets (1c-2€) were issued every year from 2014 to 2019.
4) Is euro collecting still as popular? Looks like there is less activity on this site and Eurohobby..or is it the summer holidays?
Don't think that a lot of collectors stopped collecting Euros. But I noticed, that a lot of collectors reduced coins that they're collecting. Bu sets and UNC sets are surely not as popular any more as they were before.
I'm also not quite active here, but this is because I already have long term and reliable swap partners from majority of EURO countries.
Summer holidays surely has influence on swapper's activity. This is obvious also at eBay. Summer (and also other) holidays is the best time to buy and the worst time to sell coins at auctions...
Elvis.