Problem solved, thank you, mindless, I really appreciate your assistance.
And for anyone who has the same problem:
First the copy-pasted error message I got in debug output together with the query that caused it:
(mysql): SELECT g2_Entity.g_id, g2_Entity.g_creationTimestamp,
g2_Entity.g_isLinkable, g2_Entity.g_linkId,
g2_Entity.g_modificationTimestamp, g2_Entity.g_serialNumber,
g2_Entity.g_entityType, g2_Entity.g_onLoadHandlers,
g2_ChildEntity.g_parentId, g2_FileSystemEntity.g_pathComponent,
g2_Item.g_canContainChildren, g2_Item.g_description, g2_Item.g_keywords,
g2_Item.g_ownerId, g2_Item.g_summary, g2_Item.g_title,
g2_Item.g_viewedSinceTimestamp, g2_Item.g_originationTimestamp,
g2_AlbumItem.g_layout, g2_AlbumItem.g_theme, g2_AlbumItem.g_orderBy,
g2_AlbumItem.g_orderDirection FROM g2_Entity, g2_ChildEntity,
g2_FileSystemEntity, g2_Item, g2_AlbumItem WHERE g2_Entity.g_id IN (6)
AND g2_ChildEntity.g_id=g2_Entity.g_id AND
g2_FileSystemEntity.g_id=g2_Entity.g_id AND g2_Item.g_id=g2_Entity.g_id
AND g2_AlbumItem.g_id=g2_Entity.g_id
1054: Unknown column 'g2_AlbumItem.g_theme' in 'field list'
adodb_mysql._execute(SELECT g2_Entity.g_id,
g2_Entity.g_creationTimestamp, g2_Entity....) % line 818, file: adodb.inc.php
adodb_mysql.execute(SELECT g2_Entity.g_id,
g2_Entity.g_creationTimestamp, g2_Entity...., Array[1]) % line 375, file: DatabaseStorage.class
mysqldatabasestorage.loadentities(Array[1]) % line 113, file: GalleryStorage.class
gallerystorage.loadentities(Array[1]) % line 82, file: GalleryEntityHelper_simple.class
galleryentityhelper_simple.loadentitiesbyid(6) % line 2042, file: GalleryCoreApi.class
realpath(/var/www/html/gallery2/modules/core/classes/../../..)
It is so selfexplaining that even with my little knowledge of MySQL I was able to fix this by manually adding a column 'g_theme' (as well as the colums 'g_orderBy' and 'g_orderDirection') to the 'g2_AlbumItem' table (according to definitions found in modules/core/classes/GalleryStorage/DatabaseStorage/schema/platform/mysql/GalleryAlbumItem.sql) and filling it with default values found in modules/core/CoreModuleExtras.inc.
This solved the Database Error. After that I got yet Error (ERROR_MISSING_OBJECT) message, but with help of another topic in this forum I solved that one too by flushing the cache.