Grammar-Kit: toy grammar fails with recoverWhile

I am using Grammar-Kit to create a language plugin. However, I am struggling with Grammar-Kit's poor documentation (in contrast to excellent JFlex) and with possible bugs within Grammar-Kit itself. I managed to have the language parse, but as soon as I try to add error recovery, I am at a total loss. Moreover, additional errors seem to emerge if I do white space handling in the grammar itself.

Basically, I would be satisfied if I could provide a single rule where to restart the grammar and let the parser skip tokens and retry until the number of parseable tokens exceeds a threshold.

Anyway, I think I could condense one of my many problems to the following toy grammar and example. Please help as I cannot see what I am doing wrong:

Toy.bnf:

{
  tokens = [
    space='regexp:\s+'
    id = "regexp:\p{Alpha}\w*"
    eq = "="
    colon = ":"
    brace1 = "{"
    brace2 = "}"
  ]
}
value           ::= id | object
object          ::= '{' (prop | expr) '}'
prop            ::= id ':' value             {pin = 2 recoverWhile = recover}
expr            ::= id '=' id
private recover ::= !(':' | '=' | '}' | '{')

Toy.bnf.preview:

{a=b}

This expressions parses without the pin/recover rules, but NOT as is posted above (red mark below '=', "id expected, got '='"). The value I use for pin doesn't matter.

I assume the prop rules gets pinned despite there not being any ':'.

Is it a bug or am I doing anything wrong?

Moreover, I regularly get more such problems if I include my own rules for white space. Is there any specification document about what stuff Grammar-Kit supports in {}? The three .md files from the Grammar-Kit source only provide examples, never explain anything.

Any help would be highly appreciated.

Thanks, Falk